Fix clustering sometimes displaying "Various Artists"

This fix is twofold:
1. Fixes the bug where the clustering does not select an artist name when there are some artist names appearing only once and those are at the end of the tracklist.
2. Use album artist tag, if present in the files.

Having this patch clusters will only have "Various Artists" set when there is either no artist set or all tracks are by different artists.
Merge pull request #284 from phw/fix-cluster-artist-names

Fix clustering sometimes displaying "Various Artists"