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
The ".jpg" shouldn't be there, you can choose other images (like a gif on http://beta.musicbrainz.org/release-group/3a048869-2167-4db4-9a9b-c0ec0484580f/details) as release group covers. It's true that the url with .jpg does work, but I think that's just an implementation detail of the code behind coverartarchive.org and we shouldn't rely on it.