* support for headers went into new _makeRequest method on backend base class
* support for setting backend url on initialization (rather than depending on it being on Dataset/Document objects)
* move upsert and delete methods out into distinct methods from being inside backbone sync
The two last of these pave the way for use of ElasticSearch backend standalone (both independent from Recline and independent of Backbone)
* Also remove recline.Model.backends registry as can be replaced with this more generic solution.
* This refactoring is necessitated by our need to serialize backend info for save/reload of a dataset and explorer state in #88.