Add support for music album cover art via http://coverartarchive.org/" target="_blank">http://coverartarchive.org/

The MusicBrainz! plugin can now retrieve cover art for music albums
via its http://coverartarchive.org/" target="_blank">http://coverartarchive.org/ subproject.

The data will be saved in the application folder or next to the file
depending on the system settings and added via nfo:depiction to the
nmm:MusicAlbum.