AUI-1386 - Preventing users from focusing outside the modal when it's open

The user is currently able to focus on elements behind the modal, though only when they're inside iframes. This change adds code to prevent this by changing all iframes outside the modal to have a tabIndex value of -1. It restores the tabIndex attribute when the modal is closed.
AUI-1386 - Source formatting
AUI-1386 - Prevent users from focusing outside the modal when it's open

Conflicts:
	src/aui-dialog/js/aui-dialog.js