Show radio stations in browser or as playlist

Depending on preferences of the user, we show radio stations as browsable
directories or as playlist.

Browsable directories:
* Automatic reloading on every browse request
* Not all clients support it

Playlists:
* All clients support playlists
* Slow on initial load
* Radio stations are only reloaded on mopidy startup

Default configuration enables browsable directories and disables playlists.
Merge remote-tracking branch 'upstream/develop' into radio-stations-2

Conflicts:
	mopidy_gmusic/__init__.py
	mopidy_gmusic/library.py
	mopidy_gmusic/playlists.py
fix flake8 errors
fix flake8-import-order
Add all_access config to readme

refs #34, #44
Merge pull request #49 from felixb/all_access_readme

Add all_access config to readme
Merge remote-tracking branch 'upstream/develop' into radio-stations-2
Minor fix, readme, tests

* fix crash when user is not logged in
* add radio part to readme
* add some tests to cover browsing radios
populate: Album.images

Some frontends may use this to show cover art.

Field introduced in mopidy/mopidy#314.
Async refresh

Move refresh of google music content to an extra thread.
This fixes two problems:

1. Very long startup times on small systems, like the raspberry pi
2. Remote changes to the content, like new/changed playlists
Split library and playlist refresh

* split library refresh from playlist refresh
* use some more sane method to schedule refresh
Move checkiing to one place
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