Break the navigation on two levels at a new breakpoint

We now break the navigation on two levels for a much larger share of our audience, especially laptops with screens with a resolution of 1280x800px (eg: Macbook Pro 13").

- [new] Hide science and education sections (disclaimer: dirty hack until it's done properly by @tudorraul)
- [new] New breakpoint `navigationBreakOnTwoLevels` (13 grid columns, which is 1100px)

![screen shot 2014-07-16 at 16 33 00](https://cloud.githubusercontent.com/assets/85783/3601129/a4f046ba-0cfe-11e4-800e-355d4c8c54d0.PNG)

![ ](http://gifs.joelglovier.com/dancing/advance-break-dans.gif)