Fixed nav in basic and narrow layouts

Hi Eric. You are on to a good thing here. This has saved me a few hours. Thank you for your work on this. I have fixed up some little issues.

In the basic layout the nav-collapse div needed an extra collapse class to allow the dropdown items to be visible in the collapsed state.

![Basic Collapsed Nav bug](https://f.cloud.github.com/assets/929529/70621/e8768edc-5fb5-11e2-9b77-48c4fae984a2.png)

In the narrow layout the y-overflow on .masthead .nav stopped the dropdown from displaying.

![Narrow Nav Bug](https://f.cloud.github.com/assets/929529/70622/e8c97e9e-5fb5-11e2-83eb-0bc119c64bba.png)