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)



This is functionally different from article pages and is also a different conditional check to the one used for the `content-card--byline-pic` class. Both should use the same - `content.hasTonalHeaderByline && content.hasLargeContributorImage` - as this ensures correct tonal ordering and ensures we only show byline image when there is a single contributor.