Add a 'background_art' field to Track.
Create a reasonably sensible framework for updating album art.
Add (and document) settings for Last.fm API keys.
Set up the retrieval of art from the last.fm API.
Get, serve and show album art for tracks that we can find it for.
Improve art finding logic and blur images a little on the way in.
Add a --quick option to update_background_art that skips track with art.
Increase opacity of tracks a little.

Having blurred them, a higher opacity seems to do little to harm the
readability.
Explicitly set the format and quality of background art.
Don't freak out about the occasional non-RGB image we might come across.
Don't convert images unnecessarily.
Increment has_art when skipping a track because it has art.