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)
The explanation is in the PR description, but using the android library plugin you can run jacoco smoothly, using the correct set of sources.