mutagen.id3 needs:
 * Specify all known frames
 * 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
 * Polish padding - perhaps make it client selectable

mutagen.mp4 needs:
 * To exist.
 * Clean MP4 metadata reader at
   http://svn.myrealbox.com/viewcvs/trunk/entagged-sharp/src/Mp4/Util/Mp4TagReader.cs?rev=53657&view=auto
