Inherit CaaCoverArtImage and CaaThumbnailCoverArtImage

It eases debugging using classes named CaaCoverArtImageRg and CaaThumbnailCoverArtImageRg for release group fallback

Example of log line after this change:
D: 17:08:23 Cover art image downloaded: CaaCoverArtImageRg(url=u'http://coverartarchive.org/release/06c97185-65df-4c2b-b803-428c710bedc0/2153324479-500.jpg', types=[u'front'], is_front=True) [w=352 h=350 mime=image/jpeg ext=.jpg datalen=35765 file=/tmp/picardrdYsnE.jpg]