Request CAA only if MB indicates release has caa artwork

Since 2013/01/28, web service is indicating if release has artwork
at cover art archive, use this info to prevent useless requests.
Related to http://tickets.musicbrainz.org/browse/MBS-4536
Merge branch 'ws_caa_mbs_4536' of https://github.com/zas/picard
Simplify the cover-art-archive element handling