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 that 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.

In addition it also removes the Spotify watermark.