Menu Customizer: implement previewing of menus in the Customizer. Currently not as elegant as it could be, but would require some minor core changes for improvement. Requires PHP 5.3, currently.

Also add an available menu item type JS template, to be used to improve the structure of the add-menu-item panel.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@961846 b8457f37-d9ea-0310-8a92-e5e31aec5664