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 ;)
I see that the -d is for debug, we don't want to run it on debug mode. Also in [this README](https://github.com/kt3k/coveralls-gradle-plugin) it says to just use:

```
after_success:
- ./gradlew cobertura coveralls
```

So I guess it is assuming the test data is already there so cleaning the build might not be the best idea.

I'm going to cherry pick your commit is that is fine with you.