Add support for playlist images to get_images()

[pft]

Spotify playlists may have images associated, and made available through their api.

They cannot currently be retrieved with get_images(). Parsing the URI already errors out.

ValueError: Could not parse u'spotify:user:accountname:playlist:2BCH....nws1' as a Spotify URI

[jaedb]

+1

[kingosticks]

For anyone looking at doing this, note that the difference with playlists is that they require authorisation to access.

[adamcik]

3e82e54 fixed this.