Robolectric and JaCoCo setup

This set of changes in the `robolectric-tests/` module allows to enable code coverage reports using JaCoCo.
The main problem using the `java` plugin with Robolectric is that is ~~not easy~~ impossible to make JaCoCo picking the correct source folders to generate the reports. Applying the `robolectric` plugin along with the `com.android.library` one, sources are picked correctly instead.

**NOTE:** With this configuration it seems not possible to run tests in the IDE ([@blundell's solution](http://blog.blundell-apps.com/how-to-run-robolectric-junit-tests-in-android-studio/) does not work with this setup)
I am using robolectric 2.3, that actually needs `@Config` when targeting API > 18. Given this is a test project I can see the value of using the latest plugin, but if @malmstein doesn't like it I will change it to Robolectric 2.2