Added branch support

Allows importing from a branch as well as pushing/pulling from within
the GitHub screen.
Merge branch 'development' of git://github.com/thunsaker/CloudPebble into github-branch
Fix bugs in GitHub branch support

- An empty raise is only legal in an Except clause
- Give an actual error message when pulling/pushing from/to
  branches that do not exist.
- Builds triggered automatically on push should only be triggered if
  the branch pushed to matches the branch the project uses.
Merge branch 'github-branch', with thanks to @thunsaker.