fix(download-branches): download by branch rather than stable/unstable

The download modal dialog box is now data driven based on a collection of
branches and builds.

The Twitter Bootstrap JS library has been replaced by pure AngularJS
from the AngularUI Bootstrap library. (It's so nice!)
fix(examples-tabs): Implement angular-ui tabs
refact(download-data): move download-data into its own file

Putting the download data into its own file, makes it easier to automatically
update the data when we do a release.