Use coveralls

I like using a [Coveralls][] badge for two reasons:

* Shows off good test coverage
* Visually indicates slippage in test coverage and announces it to the world

Here are two examples with varying test coverage:

* [django-model-utils][]
* [pep438][]

Steps to add coveralls support for measuring test coverage changes:

1. Install coveralls in .travis.yml file and add "coveralls" to after_success section
2. Enable coveralls for this repository
3. Add coveralls badge to README.rst file (optional)

I really like the repository setup so far: tox.ini, CONTRIBUTING.rst, Travis file, and all the other commonly-overlooked.  Really great job!

[coveralls]: https://coveralls.io/
[django-model-utils]: https://github.com/carljm/django-model-utils#django-model-utils
[pep438]: https://github.com/treyhunner/pep438#pep438