populate: Album.images

Some frontends may use this to show cover art.

Field introduced in mopidy/mopidy#314.
Add LRU cache for AA albums/tracks

The two caches provide information on tracks for `lookup()` calls,
reducing network IO and latency:

    flx@t420:~$ time mpc add gmusic:album:B6dlshnrjk4ucvcmyptc6p35pwi

    real	0m0.700s
    user	0m0.001s
    sys		0m0.001s

    flx@t420:~$ time mpc add gmusic:album:B6dlshnrjk4ucvcmyptc6p35pwi

    real	0m0.006s <-- dropped to 1/100 \o/
    user	0m0.001s
    sys		0m0.001s

Cache size is set to 1024 objects each.
It may make sense to increase size of the track cache.
Merge branch 'felixb-async-refresh' into develop
Merge branch 'radio-stations-2' of https://github.com/felixb/mopidy-gmusic into felixb-radio-stations-2

Conflicts:
	mopidy_gmusic/__init__.py
	mopidy_gmusic/ext.conf
	tests/test_extension.py
Fix merge
Merge branch 'felixb-radio-stations-2' into develop
Merge pull request #47 from felixb/browse-library
Merge branch 'populate-album.images' of https://github.com/felixb/mopidy-gmusic into felixb-populate-album.images
Merge pull request #50 from felixb/populate-album.images
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.
Increment play counts

Increment play counts for played tracks if 2/3 of track where played
Merge branch 'play-count' into all-features
Merge branch 'missing-fields' into all-features