Have fetchart grab fanart as well

I think it would be cool for the fetchart plugin to grab artist fanart and thumbnails for use in, for example, XBMC. It should save the art in the artist folder as fanart.jpg (for 16:9 images) and folder.jpg (for thumbnails of artists).

I'm sure it's pretty easy, I looked at the code, but didn't want to implement it wrong. Anyway, htbackdrops.org has the fanart, and the search would probably be this:

"http://htbackdrops.org/api/{api}/searchXML?mbid={mbid}&aid=1,5"

Beets would need to have an api key from htbackdrops, you can use TESTKEY to test it out.