Splitting _mb_request() into _mb_request() and _mb_request_raw()...

... so that the raw XML from the webservice can be returned directly.

This is important for applications that need to store the XML files like the musicbrainz-catalog https://github.com/rlhelinski/musicbrainz-catalog and for validation of the XML parsing function (parse XML, generate XML from dictionary representation, then compare to original XML).