Merge branch 'better-beets'.

Makes beets actually usable (and configurable) on Nix(OS), if you want
to use more plugins rather than just plain lookup of tracks based on
(fuzzy) string matching.

This also changes the derivation name from "python2.7-beets" to just
"beets".

* Commit summary:
  beets: Check dependencies on activated plugins.
  beets: Check plugin definitions against package.
  beets: Use audiotools backend for replaygain.
  beets: Allow to configure plugin dependencies.
  beets: Switch to using fetchFromGitHub.
  python: Add new package audiotools.
  python: Add new package discogs_client.
  python: Add pyacoustid and dependencies.
  python/mutagen: Update to upstream version 1.27.
  mp3gain: Fix output path bin directory.
  beets: Add myself to maintainers.
  beets: Update to new upstream version 1.3.9.
  beets: Move into its own package directory.