Adding high resolution cover art for Spotify

All we need to do is change the original url (http://open.spotify.com/thumb/<hash>) to one the has one of the following sizes: 60, 85, 120, 300, or 640 so it becomes http://open.spotify.com/<size>/<hash>

300 seemed most appropriate for the Sound applet.

As an additional bonus, it also removes the Spotify watermark.
Merge pull request #2782 from gillesdemey/patch-1

Adding high resolution cover art for Spotify