Support multiple artists, gets release artists directly from HTML
Tested against:
- http://www.beatport.com/release/rockin-n-rollin-the-remixes/1143465 (single artist)
- http://www.beatport.com/release/stay-the-night/1187110 (multiple artists)
- http://www.beatport.com/release/hu-dj-tools-vol-1/1372451 ('Various Artists')
Minor cleanup of argument checking, and HTML niceness