Mopidy-spotify

- 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)

- Add ``spotify/settings_dir`` config value so that libspotify settings can be
stored to another location than the libspotify cache. This also allows
``spotify/cache_dir`` to be unset, since settings are now using it's own
config value.

- Make the ``spotify/cache_dir`` config value optional, so that it can be set
to an empty string to disable caching.

- Support searches from Mopidy that are using the ``albumartist`` field type,
added in Mopidy 0.16.

- Ignore the ``track_no`` field in search queries, added in Mopidy 0.16.

- Abort Spotify searches immediately if the search query is empty instead of
waiting for the 10s timeout before returning an empty search result.

- Moved extension out of the main Mopidy project.