Changes to better support accessibility via ARIA

These changes should close #304
Hmm.  I did this at the behest of the PFWG editors - the people who designed ARIA.  I confess my understanding of the semantics of ARIA are not as good as theirs.  

As to why role=main instead of the main element, it is because role=main has well defined semantics and is already supported by assistive technologies.  the main element is not yet widely supported.

Let me go back to the PF editors and try to get a clear picture of why they think the 'main' sections should only be primary content.