Navigation (WIP)

This is a work-in-progress pull request that implements the changes discussed in #31. Here's what I have to do:

- [ ] Skin default navbar.
- [ ] Skin inverse navbar.
- [ ] Skin primary navbar.
- [ ] Skin secondary navbar.
- [ ] Skin utility navbar.
- [ ] Skin footer navbar.
- [ ] Make sure modifiers (inverse, static, and fixed) work with all navbars.

Along with navigation-specific changes, it starts to clean up some duplication inside the demo. I'll try to clean up duplication where I see it, but it could use dedicated attention in a PR.