aria-kbdshortcuts property for review - ACTION-1642

I have posted a branch with Rich's proposal for the aria-kbdshortcuts 
property:

https://rawgit.com/w3c/aria/Keyboard_Shortcuts/aria/aria.html#aria-kbdshortcuts

A couple notes:

  * I made "must" to be RFC2199 MUST.
  * Because of this, the DOM Level 3 KeyboardEvent key Values spec
    needed to be a normative reference. We've avoided having too many
    normative dependencies, and while it's ok for now, ARIA 1.1 wouldn't
    be able to advance to Recommendation until that spec does. That may
    be appropriate, but I wanted to point it out.
  * After putting the property on the four roles listed in Rich's
    proposal, I note that it inherited into one role not listed in the
    proposal: switch. That's probably fine but wanted to make sure it
    was clear where it's inheriting into.

Michael