Feature: Support for git branches

[brianwarner]

Right now Facade just analyzes whatever branch is checked out. It would be nice to be able to check out other branches directly from the UI.

When this happens, Facade will detect any commits which are in the new branch, or are missing, and do the right thing. The main thing that's needed is to detect the remote branches, and a way to check out the new selection the next time facade-worker.py runs.