Responsive secondary nav.

Desktop view with subnav:
![1403600136](https://cloud.githubusercontent.com/assets/708296/3369289/196f9b80-fb7f-11e3-8882-0b5fc3d00f12.png)

< `wide-view` with subnav
![lp-responsive-secondary-nav-gh](https://cloud.githubusercontent.com/assets/708296/3429706/c04aa87c-0058-11e4-896b-8061d0233b1e.gif)

... and to prove that it's LICEcap doing the weird background colour thing in that gif:
![1404134127](https://cloud.githubusercontent.com/assets/708296/3429707/c5272d2a-0058-11e4-8389-e0ebd43cebc0.png)