Increment play counts

Increment play counts for played tracks if 2/3 of track where played
Merge pull request #51 from felixb/play-count

Increment play counts
Browse genres

This will preload a tree of genres and try to let users find tracks with these genres.
User uploaded tracks my have genres not included in this tree!
Fix session.get_genres() compat

The previous version was not working with gmusicapi=dev.
fix test