Adds git commit message style

I proposed the idea based on [Karma Runner][k] and [Angular][a] commit styles.

Tarr, Adam, Ethan, Jeremy, and Mike discussed this and like the idea.

[k]: http://karma-runner.github.io/0.8/dev/git-commit-msg.html
[a]: https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit#