Simplify and remove AjaxManager dependency

This pull request does 2 things:

* overloads Backbone.ajax instead of Bacbone.sync as [recommended here](https://github.com/jashkenas/backbone/blob/af0a76b3fdee7887bd92cda7e041d229bb600125/backbone.js#L1275)
* removes AjaxManager dependency by using a [jQuery.Deferred](https://api.jquery.com/jQuery.Deferred/) object