Use nav for proper HTML5 ARIA role mapping

Improves upon https://github.com/twbs/bootstrap/commit/a8d95d4721e714a7e1d683e14e997faa5ccddaff by using the official mapping and proper contextual use of the nav element within the scope of ARIA roles.

http://www.w3.org/html/wg/drafts/html/master/sections.html#the-nav-element

http://dev.w3.org/html5/spec/sections.html#the-nav-element

http://blog.paciellogroup.com/2013/02/using-wai-aria-landmarks-2013/#tablex