FetchArt plugin: Support grabbing album art from MB release group

I've noticed that some of the albums in my collection don't have album art on MusicBrainz, however other releases in the release group do have album art. The Cover Art Archive has an API for [fetching art from a release group](http://musicbrainz.org/doc/Cover_Art_Archive/API) -- so here's my stab at adding that as a possible source of art.

I'm not great at Python and this is my first time coding w/ beets, so apologies if I've broken style rules here.