Initial header styles

This pull request implements the header bar:

![screen shot 2014-11-15 at 12 02 32 am](https://cloud.githubusercontent.com/assets/1168853/5057000/c1512caa-6c5a-11e4-95c3-1ac7ca643dbd.png)

It works on mobile too!

@vivekraghuram I would really appreciate if you could strictly review my use of foundation - it's my first time using it so I'm not really very familiar. Particularly, the workaround in `nav.css` to prevent the "noah5" (username) from being shown on the second screen below:

![screen shot 2014-11-14 at 11 58 43 pm](https://cloud.githubusercontent.com/assets/1168853/5057004/f798e096-6c5a-11e4-809a-1924a1b1a21c.png)![screen shot 2014-11-14 at 11 59 04 pm](https://cloud.githubusercontent.com/assets/1168853/5057007/feb3a3ac-6c5a-11e4-9b3a-54133628bb15.png)

I'm not really sure if there's a foundationy way to do it, so I just worked around it in the scss:

```scss
    .user-controls > .dropdown a.js-generated {
        display: none;
    }
```
but that seems a bit hacky. Thanks!