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.
Browse Artists and Albums

Let user browse the existing/cached library.
No network IO calls done during browse.

Directory Structure:

* Google Music
  * Albums
    * Tracks
  * Artists
    * All Tracks
      * Tracks
    * Albums
      * Tracks
Merge remote-tracking branch 'upstream/develop' into radio-stations-2

Conflicts:
	mopidy_gmusic/__init__.py
	mopidy_gmusic/library.py
	mopidy_gmusic/playlists.py
Use _lookup_*() calls for browsing

Include AA content for browsing.
Global album list is still without any AA content.
Browsing an artist which is available on AA shows all albums of artist.
fix flake8 errors
fix flake8-import-order
fix flake8 errors
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