Change schema.org mapping from MusicRecording to MusicAlbum

http://schema.org/MusicRecording is intended for individual tracks, and
although we cannot be sure with just "Audio", most library systems seem to
catalog at the album level... so switch to MusicAlbum instead.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Merge pull request #73 from dbs/schema_org_musicalbum

Change schema.org mapping from MusicRecording to MusicAlbum