Revamp menus

This commit makes .pure-menu smaller, more responsive, and easier to customize.

- Remove `.pure-menu-open`
- Remove dropdown menu CSS
- Add `.pure-menu-responsive` and `.pure-menu-toggle` to
  make responsive horizontal menus.
- Make pure menus *really* easy to customize
Merge branch 'new-menu'

Conflicts:
	src/menus/tests/manual/menus.html