A MusicBrainz release represents the unique release (i.e. issuing) of a product on
 a specific date with specific release information such as the country, label, barcode
and packaging.
A release group is used to group several different releases into a single
logical entity.
Add Grilo Media support for MusicBrainz release and release group identifiers.
Deprecate use of album identifier since it could mean release or release group id.

#136