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
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
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