Add a really simple sir trevor oembed block for embedding external content

![screen shot 2014-02-21 at 16 07 20](https://f.cloud.github.com/assets/111218/2236140/5bc56f9e-9b55-11e3-90de-d3f293db6f88.png)
![screen shot 2014-02-21 at 16 07 13](https://f.cloud.github.com/assets/111218/2236141/5bd9afd6-9b55-11e3-8070-a116acec9315.png)