Replace mediafile.py tagging with native mutagen solution.

The version of mediafile.py included from the beets project has some
problems, including, but not limited to, the following:

  * It always writes a broken USLT tag in MP3 files.
  * It writes TBPM=0 in MP3 files if the FLAC bpm tag isn't present.
  * It writes albumartist in the ID3 TXXX:Album Artist tag (a
    deprecated foobar2000 behavior).
  * It supports some other unusual/deprecated tags.

mutagen's EasyID3 implementation fixes all these issues, supports a
bunch of "standard" tags not supported by mediafile (conductor, asin,
additional MusicBrainz tags, etc.), and the code needed to implement
it is a lot smaller than mediafile.py.

It's also easily extensible, and this change takes advantage of that
to add support for some common tags that EasyID3 doesn't support out
of the box, most importantly albumartist (TPE2).
Stub out unused 'key' parameter.
Support for TRCK and TPOS x/y-style tags.