Plugin for Beatport Metadata in Autotagger

This adds support for querying Beatport's catalogue during autotagging. Beatport's repository can be advantageous if a user's collection has a lot of electronic music releases in it, for which both Musicbrainz and (to a lesser degree) Discogs sometimes yield no matches. Inspiration came from issue https://github.com/sampsyo/beets/issues/124
The API is documented here: http://api.beatport.com.