browser github branches

Two-column explorer in breadcrumb area for #353.
![2014-11-02_2140](https://cloud.githubusercontent.com/assets/45380/4878141/d664d2c4-6302-11e4-975e-495636418bf4.png)

Possibly could be applied to gists as well, but the API seems a bit different.
Rather than `thing`, you could call this a `ref` too. I'm guessing you were torn between `ref` in the URL (the name) and the `ref` object that GitHub is returning from the API.