Layout hinting - experiment

This PR introduces some simple logic and styling to experiment with layout hinting options within article elements, such as images and videos.

We are using a live feature article on the rescue from Antarctica to demonstrate the possibilities. The feature is behind a self destructing switch, when the time frame ends I will delete the code and the article will gracefully degrade back to a normal looking article with in-line embeds.

Along with Sean Clarke et al we have agreed on a namespace for url slugs ```/-st-``` to force any article to NGW and kick in this styling.

The end goal here is for composer to handle layout hinting natively. We have been told this will arrive some time in March/April if this goes successfully.

CODE URL: world/2014/feb/26/-sp-rescue-from-antarctica


### Screenshots

#### Full width main pic
![screenshot from 2014-02-27 16 43 52](https://f.cloud.github.com/assets/80089/2285308/fee9d9e0-9fd2-11e3-94e9-60013d4e6fd2.png)



#### Full width videos
![screenshot from 2014-02-27 17 05 45](https://f.cloud.github.com/assets/80089/2285311/03338ba4-9fd3-11e3-9c1e-ae97d91d1894.png)



#### Full width images and pull quotes![screenshot from 2014-02-27 16 44 20](https://f.cloud.github.com/assets/80089/2285313/067bb05c-9fd3-11e3-97c3-9fad2c9e13d6.png)

/cc @mattosborn @rich-nguyen @kaelig @ironsidevsquincy @commuterjoy Please can you review asap.