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.
`ref` is already the `ref` of the requested tree... i've renamed it `nav_ref`, hopefully this makes it a little more self-documenting!