Refactor admin box JavaScript to use Backbone views

[scribu]

Over time, the JS that drives the metabox has transformed into typical jQuery spaghetti code.

Since WordPress 3.5 bundles Backbone.js, we should take advantage of it, or at least of the structure that Backbone.View provides.

Using Backbone models and collections would require altering the AJAX responses significantly, which I'll leave for another time.

Guide: https://github.com/kjbekkelund/writings/blob/master/published/understanding-backbone.md