start working on "grouping"

TIT1 (ID3v2) and @grp (mp4 metadata) are "commonly" used to for grouping.
This commit adds the mp4 support pulled from my old fork.
add grouping info for mp3

This should be all thats necessary, as the 'infrastructure' was added in the
previous patch.
oops. remove a stray printf()
remove a debug line
closes #12, closes #51