NoClassDefFoundError org/slf4j/LoggerFactory;

HI,
Although i am getting this error after including aar in a project but its only java stuff.

I have a project called logging which i wanted to use in another project called Vader. So i made an aar of it.
The build.gradle file for logging is as follows :

apply plugin: 'android-library'
apply plugin: 'maven'

android {
    compileSdkVersion 19
    buildToolsVersion '19.1.0'
    sourceSets {
        main {
            manifest.srcFile 'AndroidManifest.xml'
            java.srcDirs = ['src']
            aidl.srcDirs = ['src']
            renderscript.srcDirs = ['src']
            res.srcDirs = ['res', 'res-flags']
            assets.srcDirs = ['assets']
        }
    }

    defaultConfig {
        minSdkVersion 14
        targetSdkVersion 19
        versionCode 1
        versionName '1.0'
    }

    lintOptions {
        abortOnError false
    }
}

dependencies {
    compile 'org.slf4j:slf4j-api:1.7.7'
    compile 'com.squareup:tape:1.2.2'
    compile 'com.github.tony19:logback-android-classic:1.1.1-2'
    compile 'to.talk.external:myjson:2.2.4'
}

def coreAarFile = file('build/logging.aar')
artifacts {
    archives coreAarFile
}

uploadArchives {
    repositories {
        mavenDeployer {
            repository(url: "http://ci.aws.talk.to:8081/nexus/content/repositories/snapshots",
                       authentication: [
                               userName: 'deployer',
                               password:'qwedsa'
                       ])
            pom.groupId = 'to.talk.aragorn'
            pom.artifactId = 'logging'
            pom.version = '0.5-SNAPSHOT'
        }
    }
}

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After running build in gradle it generated an aar file with name : logging-release.aar which i place in the lib folder of my vader project and add the line :
    compile(name: 'logging', ext: 'aar')
in the build.gradle of the project.
Now i am using one of its function in my project. Although the project compiles perfectly but it gives the following error on runtime :

On Running i am getting the following error :

06-22 18:30:36.797  16634-16634/co.riva.vader E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: co.riva.vader, PID: 16634
    java.lang.NoClassDefFoundError: Failed resolution of: Lorg/slf4j/LoggerFactory;
            at to.talk.logging.LoggerFactory.<clinit>(LoggerFactory.java:37)
            at to.talk.logging.LoggerFactory.getTrimmer(LoggerFactory.java:71)
            at co.riva.vader.HomeActivity.<clinit>(HomeActivity.java:60)
            at java.lang.reflect.Constructor.newInstance(Native Method)
            at java.lang.Class.newInstance(Class.java:1606)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5254)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
     Caused by: java.lang.ClassNotFoundException: Didn't find class "org.slf4j.LoggerFactory" on path: DexPathList[[zip file "/data/app/co.riva.vader-2/base.apk"],nativeLibraryDirectories=[/vendor/lib, /system/lib]]
            at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)
            at java.lang.ClassLoader.loadClass(ClassLoader.java:511)
            at java.lang.ClassLoader.loadClass(ClassLoader.java:469)
            at to.talk.logging.LoggerFactory.<clinit>(LoggerFactory.java:37)
            at to.talk.logging.LoggerFactory.getTrimmer(LoggerFactory.java:71)
            at co.riva.vader.HomeActivity.<clinit>(HomeActivity.java:60)
            at java.lang.reflect.Constructor.newInstance(Native Method)
            at java.lang.Class.newInstance(Class.java:1606)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5254)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
    Suppressed: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
            at java.lang.Class.classForName(Native Method)
            at java.lang.BootClassLoader.findClass(ClassLoader.java:781)
            at java.lang.BootClassLoader.loadClass(ClassLoader.java:841)
            at java.lang.ClassLoader.loadClass(ClassLoader.java:504)
            ... 18 more

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Even though build.gradle inside loggin has the entry for compile 'org.slf4j:slf4j-api:1.7.7'
Still when the aar is included its giving error.
What could be the reason for this. How can i correct it.

Thanks