some tweaks for `generate_tracklist` and `sort_tracklist`
some tweaks for `generate_tracklist` and `sort_tracklist`

sort_tracklist:
-	can now specify tracks as Path::Class::File's
-	now uses the tag sortkey, if it exists
generate_tracklist:
-	handle a directory as well as an album name
-	don't demand that tracks exist (just that they end in .mp3)

barefootcoder/music,barefootcoder/music,barefootcoder/music