Include context when updating GitHub status

Replace "default" with "hound" in status output.

![screenshot 2014-12-17 17 06 01](https://cloud.githubusercontent.com/assets/154463/5482045/0367bbf4-860f-11e4-8786-3c4edca61959.png)

I don't think having Hound specific code in a `lib` class is a good idea.