Fix PHP include and require calls.

Turns out you need to add quotes around the path to the file you want
to include/require.
Start boilerplate for nav bar.

Elected to go with Tim Pietrusky's "Select" schema:
http://css-tricks.com/responsive-menu-concepts
Add universal border-box box sizing.

Paul Irish says it's OK:
http://www.paulirish.com/2012/box-sizing-border-box-ftw/