Restructure

Turns it more into a boilerplate. Solves #3.

- oak is now app. Makes it easier to guess that it's the main app folder and not a "library".
- app currently only has an `index.html` + styles. Eventually could have more stuff. Also, some starter templates.
- demo is now styleguide. You use it to get a quick overview of all components. Plus for testing during dev.