Mongoid support, confirmations, code cleanup, and jQuery UI gem usage

Sorry for the big blob of commits. I can make separate pulls against master if you like. This set of commits does four things:

* Adds Mongoid support for assets, with tests.
* Add deletion confirmation support.
* Rips out some unnecessary things, like the Ajax loading of the application.js file and some unnecessary double-instantiating of the uploader widget.
* Use the jQuery UI gem instead of the bundled version of jQuery UI. The uploader seems to be working ok with the latest jQuery UI.

I've been using my fork in production for a while now on both Mongoid and ActiveRecord databases and it's working pretty well.