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`
The problem is that those args aren't passed in the sync method...your success/error callbacks get `(model, response, options)` from each different method (create, fetch, etc.) See https://github.com/jashkenas/backbone/blob/master/backbone.js#L453