Snap heights

* Make iframes for embeds match the height of their containing item
* Don't let their height go below a 2:1 aspect ratio; eg for letterbox layouts such as on mobile or in a single-item slice (example 2)
* Don't let their height to go above 400px

Example 1:
![screen shot 2014-10-27 at 17 04 52](https://cloud.githubusercontent.com/assets/1147913/4799564/79925f4c-5e1d-11e4-963f-9fc007429f04.png)

Example 2:
![screen shot 2014-10-27 at 17 05 11](https://cloud.githubusercontent.com/assets/1147913/4799572/8a9ff4ca-5e1d-11e4-856c-db50588a723a.png)