Mopidy-spotify

- 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.