Spotify Does Not Work

[sammerry]

The spotify plugin does not work. A permissions change removes its access to the cache directory

INFO     Creating dir /var/cache/mopidy/spotify
ERROR    Unhandled exception in SpotifyBackend (urn:uuid:41aa8442-d6ec-4891-8917-c3f2cdb80a62):
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/pykka/actor.py", line 192, in _actor_loop
    self.on_start()
  File "/usr/lib/python2.7/dist-packages/mopidy_spotify/backend.py", line 54, in on_start
    self._session = self._get_session(self._config)
  File "/usr/lib/python2.7/dist-packages/mopidy_spotify/backend.py", line 76, in _get_session
    session = spotify.Session(self._get_spotify_config(config))
  File "/usr/lib/python2.7/dist-packages/mopidy_spotify/backend.py", line 104, in _get_spotify_config
    spotify_config.cache_location = ext.get_cache_dir(config)
  File "/usr/lib/python2.7/dist-packages/mopidy/ext.py", line 75, in get_cache_dir
    path.get_or_create_dir(cache_dir_path)
  File "/usr/lib/python2.7/dist-packages/mopidy/internal/path.py", line 31, in get_or_create_dir
    os.makedirs(dir_path, 0o755)
  File "/usr/lib/python2.7/os.py", line 157, in makedirs
    mkdir(name, mode)
OSError: [Errno 13] Permission denied: '/var/cache/mopidy/spotify'

This PR fixes it as well as alsa audio.
#20