Implement static top nav bar

This is the next step in porting the old bootstrap code from 1.x to 3.0.
Everything that starts with `.navbar-default`, I'd nest under `.navbar-default` so you don't have to repeat yourself:

```scss
navbar.navbar-default {
  .navbar-text {
    // styles
  }

  .navbar-nav > li > a {
    // styles
  }
}
```

etc.

Docs: http://sass-lang.com/guide