Add cobertura and coveralls support

This was mentionned during https://github.com/JCAndKSolutions/android-unit-test/issues/41

To enable it : go to [coveralls](https://coveralls.io/), sign in, and add the repo.

Usually, what I do that I enable the coveralls locally on my machine after that. 
```bash
export CI_NAME=travis; export CI_BUILD=1341213  //we don't care about the build value
export COVERALLS_REPO_TOKEN=<see coveralls site for your repo>
./gradlew cobertura coveralls 
```

First build may fail but second will work.

Then
If you can't find the source code when you enable coveralls (SOURCE FILE DETAILS page), then : 
Fill the field : Git repo root directory=<server path to >/android-unit-test   <--- DO NOT ADD A FINAL SLASH

I let you add the coveralls badge and see the result of coverage up to now. It's gonna be a surprise ;)
Why are you adding jcenter when mavencentral has them?