Support artist-credits nested in recordings nested in releases

... What a mouthful!

Musicbrainz lets you do things like this:

http://musicbrainz.org/ws/2/release/94cadd69-54b5-40ee-8e53-7fbbd82654d3?inc=artist-credits+recordings+media

The library currently didn't support this correctly. It still mostly doesn't, I just fixed the one place I needed it to work in, but you could apply this elsewhere too I think.