Add branching capabilities

NB: I haven't tested this stuff because I can't.

This adds the ability to checkout a specific branch.
For example, if I wanted to checkout the `3.0.0-wip` branch of bootstrap instead of the master branch I could go to `5minutefork.com/twitter/bootstrap/3.0.0-wip`

This *should* not break existing URLs. So `5minutefork.com/twitter/bootstrap` *should* still work the same as it currently does.