Add coveralls to provide more information about test coverage

[Coveralls](https://coveralls.io/) is a third-party service (free for open-source) that provides detailed information about test coverage for your app. It uses `simplecov` so it is essentially the same thing as the simplecov data that you guys have been generating with the exception that Coveralls is hosted online and provides a nice badge that you can put in your README to show what the test coverage % is.

Similar to Travis, the only thing that you should have to do to get this going is sign up for a free account on [Coveralls](https://coveralls.io/). @AlexPatriquin will have to do this. Then, once registered, enable Coveralls for this repo and you should be good to go.