Walkthrough UI

Sketch of an experimental walkthrough UI. Most of the code is refactoring DOM so that the entire walkthrough is implemented as a CSS :target + sibling system. Only for style editor atm, source editor needs more work before it can take a walkthrough I think.

![walkthrough](https://cloud.githubusercontent.com/assets/83384/2948469/89313e68-da02-11e3-8bf3-4602091614f3.gif)

### Remaining

- [x] design review @samanpwbb 
- [x] localStorage or other flag for triggering walkthrough on first run
- [x] copy/content/angle review