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`
Instead of extending the original `options` object and risking the possibility of overwriting `xhr`, the `options.success` and `options.error` callbacks should just take `data`, `status`, `headers` and `config` as parameters.

`options.success(data, status, headers, config);`