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 main problem is how the JaCoCo plugin behaves differently if used along with the `java` or `android-library` one. I could not find a solution for make it picking the sources from the android module using the `java` plugin. Crossing your solution with [this example](http://raptordigital.blogspot.co.uk/2014/08/code-coverage-reports-using-robolectric.html) I found a way to make it working.