add set_parser()

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). 

EDIT by JonnyJD:
I updated the title so this can be found easier later on.