modified ARIA mapping for header and footer elements.

Hi all,

I have modified [1] the ARIA mapping for the header and footer elements
after recent face to face discussion with Mozilla acc engineers and NVDA
screen reader implementer who agreed that converging on the webkit
implementation for these elements would be a good thing and lead to
interoperable implementation of accessibility info.

details in bug: https://www.w3.org/Bugs/Public/show_bug.cgi?id=21164

Please review at your leisure, feedback welcome!

[1]
https://github.com/w3c/html/commit/6c5c62dbc4c6674d41b263c4aea6659103995b8f
https://github.com/w3c/html/commit/1f21a4350491fc4309a7b25c615472cc8e670902(added
a word I missed in initial commit)

-- 
with regards

Steve Faulkner