Use a Backbone Model/View pair to render the featured artwork

While this is undeniabley more code, the coupling between the data and view logic has been reduced:

* Data setting happens exclusively within the `artwork` model instance
* Rendering and view state changes happen exclusively within the `featuredArtwork` view instance
* Changes to `artwork` automatically update the view via Backbone events
* Detect when the gallery is first shown using the presence of `artwork.id` instead of a DOM lookup

Because Backbone has hit 1.0 since WP 3.5, we've done a couple other things:

* De-register the built-in version of Backbone.js and use 1.0 instead
* De-register the build-in Underscore.js and use [Lodash.underscore](http://lodash.com/) (an optimized equivalent) instead