Fixes #4964. Adds a statusCode object together with a new statusCode method on the...

Fixes #4964. Adds a statusCode object together with a new statusCode method on the jXHR object (deferred behaviour). They accept a map of statusCode/callback(s). Callbacks are fired when the status code of the response correponds to the key (as a success or an error callback depending on how the request completed). Unit tests added.