Add NBS sample application

This solves #7. The sample application is the basic example application in the gradle examples archive available at the [Android Tools Website](http://tools.android.com/tech-docs/new-build-system). RoboSpock tests are in a separate test sub-project and use the [gradle-android-test-plugin by Novoda](https://github.com/novoda/gradle-android-test-plugin). This way tests can be run using the command line or Android Studio (I haven't tried with other IDEs). 

Side note: I gave Square own gradle-android-test-plugin a try but couldn't manage to make it work.