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
If you run this in your branch, git grep will of course find the strings you've just added. On master, the only matches are in NEWS.txt and picard/util/textencoding.py. In both cases, "fallback" is [correct](http://www.merriam-webster.com/dictionary/fallback) because it's used as a noun there, "to fall back" is the [verb](http://www.merriam-webster.com/dictionary/fall%20back) form.