Add Support for External Libraries in Gradle Build File

1. Create a directory in the Android project for keeping the external library .jar files, e.g. libs
2. In build.gradle, add

android {
    ...
    dependencies {
        compile fileTree(dir: 'libs', include: '*.jar')
    }
    ...
}

This causes Gradle to include all the .jar files in the directory libs in the build.

Add the Apache Commons Library

1. Download the desired Apache Commons library from http://commons.apache.org/
2. Place the main .jar file of the library in libs
3. Build as usual (./gradlew assembleDebug)