Mopidy-spotify

- Add support for browsing playlists and albums. Needed to allow music
discovery extensions expose these in a clean way.

- Fix loss of audio when resuming from paused, when caused by another Spotify
client starting playback. (Fixes: 2, PR: 19)

- Switch to new backend API locations, required by the upcoming Mopidy 0.19
release.

- Wait for track to be loaded before playing it. This fixes playback of tracks
looked up directly by URI, and not through a playlist or search. (Fixes:
mopidy/mopidy675)

- Change requirement on pyspotify from ``>= 1.9, < 2`` to ``>= 1.9, < 1.999``,
so that it is parsed correctly and pyspotify 1.x is installed instead of 2.x.

- Require Mopidy >= 0.18.

- Change ``library.lookup()`` to return tracks even if they are unplayable.
There's no harm in letting them be added to the tracklist, as Mopidy will
simply skip to the next track when failing to play the track. (Fixes:
mopidy/mopidy606)

- Added basic library browsing support that exposes user, global and country
toplists.

- Change search field ``track`` to ``track_name`` for compatibility with
Mopidy 0.17. (Fixes: mopidy/mopidy610)