Content cards

Creates an endpoint for article-embeds ( 'content cards' ) 
* Returns jsonified html / and actual html 
* Styles:

Examples

Review:
![review](https://cloud.githubusercontent.com/assets/986155/5033098/f1025b82-6b5a-11e4-94a8-82bfad548ac8.png)

Comment: 

![comment-card](https://cloud.githubusercontent.com/assets/986155/5033149/5e899544-6b5b-11e4-93f9-830f876e1514.png)



It still needs an explicit cache header (even if it is `no-cache`).

The behaviour of a missing cache header is not defined so anything can cache it if it wants to. This has caused us problems with tools before.