Add owner of playlist to the download information

I´ve found that when you are not the owner of a playlist, but a follower you cannot download it, it fails because it cannot find the playlist at the URL https://api.spotify.com/v1/users/raulcumplido/playlists/4LIpFOsgMFanDAvsNOPy1P because the playlist is not from my user:
```
Downloading playlist 'Dover' (id=4LIpFOsgMFanDAvsNOPy1P)
Traceback (most recent call last):
  File "/Users/raulcd/Envs/spotify-onthego/bin/spotify-onthego", line 9, in <module>
    load_entry_point('spotify-onthego==0.0.1', 'console_scripts', 'spotify-onthego')()
  File "/Users/raulcd/Projects/spotify-onthego/onthego/scripts/cli.py", line 22, in download_playlist
    for track_name, artist in spotify_client.iter_tracks(args.playlist):
  File "/Users/raulcd/Projects/spotify-onthego/onthego/spotify/auth.py", line 147, in iter_tracks
    for item in self.spotify.user_playlist(self.spotify_username, playlist_id)["tracks"]["items"]:
  File "/Users/raulcd/Envs/spotify-onthego/lib/python2.7/site-packages/spotipy/client.py", line 290, in user_playlist
    return self._get("users/%s/playlists/%s" % (user, plid), fields=fields)
  File "/Users/raulcd/Envs/spotify-onthego/lib/python2.7/site-packages/spotipy/client.py", line 98, in _get
    return self._internal_call('GET', url, payload, kwargs)
  File "/Users/raulcd/Envs/spotify-onthego/lib/python2.7/site-packages/spotipy/client.py", line 85, in _internal_call
    -1, u'%s:\n %s' % (r.url, r.json()['error']['message']))
spotipy.client.SpotifyException: http status: 404, code:-1 - https://api.spotify.com/v1/users/raulcumplido/playlists/4LIpFOsgMFanDAvsNOPy1P:
 Not found.
```
You can fix manually by modifying the file on ```~/.local/share/spotify-onthego/credentials.json``` and the ```username``` but the proposed fix solves the issue.