Refactor: add XHR params to options argument passed to success/error callbacks

See https://github.com/adrianlee44/ng-backbone/issues/7

This exposes the HTTP status code and headers on `options.xhr`
As it stands, `options.xhr` is the same $http promise that gets returned by `ngBackbone.sync`, anyways. You already have access to it, so the idea was to make `options.xhr` more useful.

In the $.ajax world, `options.xhr` and the returned value are a `jqXHR` object, which combines a promise interface with the normal XHR propertis.