Read year on Musicbrainz lookups

Add code for reading the year when looking up releases from Musicbrainz.
Added a test-case (musicbrainztest-fulldata), where the date
is "2011-04-29", which should be parsed as only "2011".

Also, while adding the test case, I ran into a bug, the titles
and track artists were always taken from the recording, while
ones from the "release credits" are preferred, fixed this as well,
and updated the other tests to reflect the fixes.