Twenty Twelve: add support for toggling the navigation menu in small screens. See #19978.

Triggered when a viewport is under 600 pixels wide. This functionality depends on JS being enabled, and will naturally fall back to a normal, expanded menu for a client without JS.

Props iandstewart for the original JS file and concept.


git-svn-id: https://core.svn.wordpress.org/trunk@20007 1a063a9b-81f0-0310-95a4-ce76da25c4cd