Added support for multiple artists & album images

Program now uses first artist if there are more than 1 (this should
probably be changed to a "x / y / z" format in the future), and uses the
first image if there is more than one (should add check for "primary"
image). Also, the Discogs API locks you out after 1000 requests/24
hours/IP address. That's kinda lame. I will have to explore workarounds.