Doveralls support for test coverage tracking

Dub currently has quite an extensive regression test suite, but it seems like the unit test coverage isn't quite what it could be. Using coveralls could be a great way to keep track of that coverage, and make sure that it doesn't go down.

Here's what [Dub's results](https://coveralls.io/r/ColdenCullen/dub) look like on my fork.

If you do decide to merge this, you'll need to [enable this repo in Coveralls.io](https://coveralls.io/repos/new).