Embed bootstrap-sass (locked at version 2.3.2.2).

This is a replacement pull request for #1733 that fixes the issues with the original and passes all tests cleanly.

The big difference is that while the original embedded the scss files from bootstrap-sass, it didn't embedded the lib code that was necessary for all of that to work.

I've also namespaced the bootstrap files (they're all inside the respective rails_admin directories) so that they don't interfere with a Rails app using a newer version of bootstrap by being globally available in the root include paths.