refactor(app): utilize bootstrap-sass and html rewiring

I disliked having the bootstrap scss files copied into my apps folder, so I reused my approach from `generator-ember` to use a bower package instead.

- Bootstrap plugins and angular modules are now written using the wiring
  helpers.
- Instead of copying the bootstrap files, a bower package is now used if compass
  is selected.
- The bundled bootstrap version has been upgraded to 2.3.1.

/cc @sindresorhus @btford

One thing I just noticed is that jQuery and the additional Angular modules are now within a build block, which prevents cdnify from doing its magic. Wouldn't it make sense to have an option in `generator` to disable the build block generation around appended files?