Spotify support

Spotify now supports an [embeddable player](https://developer.spotify.com/technologies/widgets/spotify-play-button/) and thought it would be a good idea to add support for Nikola, since It's quite a popular service and relatively easy to use.
![spotify](https://f.cloud.github.com/assets/553651/2301557/9cf3af52-a14b-11e3-9fd7-4a876c1a7827.png)