Fix video expandable trail

- [New] Heading level option in heading trail fragment
- [Fix] Heading level is always h3 in story package (we had a mix of h3 and a few h2)
- [Fix] Always show the experimental trails (with the heading on top of the image instead of on the right) in story package

## Before

![screen shot 2013-08-21 at 17 04 03](https://f.cloud.github.com/assets/85783/1002333/57c4a326-0a7b-11e3-85ec-54d71fdb08ab.PNG)


## After

![screen shot 2013-08-21 at 17 03 13](https://f.cloud.github.com/assets/85783/1002324/3cbd1374-0a7b-11e3-8043-d19f1cb8c558.PNG)