PICARD-418: fallback to using release group cover

It adds the option to use release group cover art from CAA.
It is required to enable CAA and release group fallback is disabled by default

http://tickets.musicbrainz.org/browse/PICARD-418
http://tickets.musicbrainz.org/browse/PICARD-53
Is there any chance to turn that 203 into a 404 (problably via some attribute of `http`)? That would make this part easier to understand.