acoustid: Retrieve additional keys
Add support for album_disc_number, publication_date and track_number tags.
- I have updated the tests. Two examples changed (ludovico_einaudi_primavera and radiohead_paranoid_android). And now the first result of ludovico_einaudi_primavera is not Ludovico Einaudi anymore. Strange...
- In some results, the publication date only has a year. So I set month and day to 1. I don't know if there is a proper way to deal with it.