MusicBrainz support

This is the rest of the MusicBrainz support for XBMC. It's pretty complete now I feel. It has hard-coded the universal music scraper in there right now, but I need to demise that - I'm working on a small restructure & tidy up to do that. But basically this shouldn't change besides for your feedback now.

It seems to show up the TagLib changes in this stream on GitHub as well. Not sure how I can remove those from this stream.