Add `css` and `js` includes`

This allows me to not have to worry about the source directory for
assets changing. In addition, if I move static (or at least globally
used assets) to a different site -> no worries! :D
Move all css, js, and images to `assets` directory
Fix css include

Turns out it needs quotes around it.
Move header and footer to includes
Add Google Analytics tracking
Add 'a:focus' wherever 'a:hover' is used
Rename 'stylesheet.css' to 'slate.css'