mutagen.id3 needs:
 * Detect tags in non-beginning locations, particularly if multiple
   separate version tags exist at the beginning of the file
 * Perhaps implement tag merging - "intelligently" merge information
   from all known tag formats
 * Some test cleanup

mutagen.apev2 needs:
 * General cleanup to fit the basic structure of the rest of Mutagen.

Profile Hotspots:
 * __determine_bpi takes 84% of TMP3,TEasyID3; BitPaddedInt takes 46%.
 * OggPage.replace takes 43% of Ogg tests; OggPage.write takes 25%.
 * insert_bytes/delete_bytes is still a hotspot but is much faster.