Added Codecov Support

I've been chatting with @jfairley about working with [codecov](http://codecov.io/), so I figured to add the support so you can see how this package works. You can see the report (from this pr) [here](https://codecov.io/github/stevepeak/org.immutables?ref=be0337bf3a39e902f1b426f4794c8e2b36d7ecd8).