fix tests after merge
Don't crash on missing fields

I found some crashes after fuzzing with strange and promiscouse mpd clients. ;)
Looks like connection is much more stable now.
more tests
Fix browsing AA artists
Fix crash browsing artists w/o albums
Merge pull request #55 from felixb/fix-tests

fix tests after merge
Merge pull request #52 from felixb/missing-fields

Don't crash on missing fields
Merge pull request #56 from felixb/fix-browse-aa-artists

Fix browsing AA artists
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.