Add SoundCloud embedding

Works on urls like:
- soundcloud.com/:username
- soundcloud.com/:username/:track
- soundcloud.com/:username/sets/:playlist
- soundcloud.com/groups/:groupName

Some examples
![selection_014](https://cloud.githubusercontent.com/assets/426222/5498509/31168d84-8720-11e4-9111-1e17abd47f78.png)
![selection_016](https://cloud.githubusercontent.com/assets/426222/5498547/b38bbcc6-8720-11e4-8c19-199976b49dd5.png)