Navigation

This is a ~~work-in-progress~~ pull request that implements the changes discussed in #31. Here's what I have to do:

- [x] Skin default navbar.
- [x] Skin inverse navbar.
- [x] Skin primary navbar.
- [x] Skin utility navbar.
- [x] Skin footer navbar.
- [x] Make sure existing styles match Canon exactly. There are currently a bunch of small discrepancies.
- [x] Make sure modifiers (inverse, static, and fixed) work with all navbars.
- [x] Make sure responsive styles work with all navbars.

Here's the basic approach:

1. Skin `navbar-default` with the colors from the primary nav (not font size, weight, or anything else), and skin `navbar-inverse` with the colors from the utility nav. Once you have a light and a dark skin, the special navigation types need very few styles. This helps avoid specificity wars.
2. Implement a `.navbar-height` mixin that adjusts the vertical margin and/or padding of `navbar-default`. I've tried to be careful to only set styles that need to be adjusted.
3. Implemented one-off adjustments for each special navigation as necessary. This includes items like font size, font weight, or font family.

Along with navigation-specific changes, I started 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.