YouTube shortened links not processed

If a user creates a link to YouTube content using the shortened URL, e.g. `http://youtu.be/JrtWhFtjXTA` the OAE preview does not embed the video.

![screen shot 2014-07-09 at 9 52 12 am](https://cloud.githubusercontent.com/assets/1731910/3525132/42db6d44-0770-11e4-8627-e450ac0c0f6a.png)

The same video, when created using the full URL (`https://www.youtube.com/watch?v=JrtWhFtjXTA`), previews fine.

![screen shot 2014-07-09 at 9 53 14 am](https://cloud.githubusercontent.com/assets/1731910/3525161/7b160c78-0770-11e4-8edf-958c0e458561.png)


Should we verify that `youtu.be` is followed by something to avoid `http://youtu.be` or `http://youtu.be/` being processed by this?