Initial version of musicbrainz provider

Here's an initial stab at MusicBrainz/CoverArtArchive.org support.

Your Google provider is providing image dimensions to the Image class when you return your results to cover.py. The response from CoverArtArchive.org does not include dimensions, so I don't know what you want to do about that.

In a horrible act of hackery, I'm simply giving the likely false dimensions of 500px x 500px just so that cover.Image won't complain.

Let me know how we should be handling that, and I'll make the appropriate changes.