Add WAI-ARIA roles

This improves accessibility for screen readers.  This is a best-effort,
I probably have things wrong and need feedback.

Currently implemented:

* Roles for various landmarks.
* Roles for document structure.
* A combobox role for the search field.  I'm not sure if this one is
  right.  The OS X Accessibility Inspector claims I've hooked it up
  correctly but I can't get VoiceOver to follow focus.  This may be due
  to my complete inexperience with VoiceOver.