Navigation module

[cesutherland]

This will be the first big module after forms.

We want to wrap up the top-level navigation from the mocks in our own navbar module. Take a look at https://github.com/twbs/bootstrap/blob/master/less/navs.less and https://github.com/twbs/bootstrap/blob/master/less/navbar.less.

Before starting this we'll want to take stock previous progress. After we've got this we'll want to apply the theme to at least one of humble's open source project pages.