Organize assets

This splits JS and CSS into separate files/folders.

The JS structure will probably change, since we'll probably end up using require.js. But this is probably a useful intermediary step.

Let me know if you prefer a different sort of CSS split. I was thinking of the following:

* One file for the universals (base.css.scss), e.g. headers, paragraphs, buttons, etc. -> stuff that can show up anywhere on the site
* Mixins file
* Directory/files for each of the pages