Run tests with coverage

You can see the coverage report in the log, but you can also use Coveralls. It's free for open source projects and you get reports like this:

https://coveralls.io/builds/679321

PyGitUp has a respectable 79% coverage.

1. First, add the repo at https://coveralls.io/repos/new
2. Edit README and change the two `hugovk` to `msiemens` for the badge to work
3. This'll trigger a build and get the first coverage reports for the badge