Album artist/primary artist scrobbling

Instead of scrobbling multiple artists as a joined string of artists, scrobble only the first artist. If there is an album artist that is neither "Compilation", "Various Artists" nor "Split" (case insensitive), the first artist of the album artist will be scrobbled. Else, the track's first artist will be. Works well with mopidy-spotify.

See this issue for more information.