Added support for Discogs web service
The new Discogs support seems a lot more up to date and complete than
the MusicBrainz interface. However, the API has no means of distinguishing
between albums and singles. To get around this, a check is performed to see
if a release is in the albums list in the Discogs web page (not from the
API)

It still has issues. I have a list of strange output which I'm going to
investigate further...