Sass updates

Moved core styles into its own "core" directory inside the sass assets
directory.

The Foundation base has been updated, and the topbar now looks proper on
firefox.
Moved topbar markup generation into a new Navbar class.

This has the benefit of being able to render dropdowns for the navbar.
I'm still not sure if the Menu helper needs to exist by itself, and why
it can't just be merged with the Navbar helper.