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* The Twitter Bootstrap JS library has been replaced by pure AngularJS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!)

@tbosch and @matsko - can you take a look at this to make sure I haven't broken anything?

* There is some "pulse" feature that doesn't seem to be turned on.  This is the only thing that is still requiring JQuery. It would be nice to remove both.
* It would be nice to move to BootStrap CSS v3.x, currently we are stuck with v2.0.x, but I didn't have the spare brain capacity or time to start we working the markup.
* We need to fix up the build script for this site, so that it updates the `BRANCHES` array in `js/homepage.js`.  The `branch` property of each object in `BRANCHES` array should match the `branchVersion` in `package.json`.