Adding headline container

Adding a new container for the days headlines.
Allows for standard and big size stories only.

Logic is:
* 2 standard stories per column
* 1 big story per column
* Max 4 columns

# What?
### mobile
![mobile](https://cloud.githubusercontent.com/assets/31692/3746844/fb20b796-17c5-11e4-8339-35194ea4a0d5.png)

### desktop
![desktop](https://cloud.githubusercontent.com/assets/31692/3746842/f5ff476e-17c5-11e4-97c4-260697dffc67.png)

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![Illusions](http://static.paulboxley.com/2014/monument_valley_penrose.gif)
We had to fix the data structure early on, whereas the role of groups is always evolving. It's a trade-off; it provides flexibility and decoupling (we don't want the tool to impose, or even know, the meaning that groups might have in templates) at the cost of being a bit obscure.  But yes, there could be abstractions to make it more readable in templates.