use nav+ul element for "you are here"

As described in the common idioms of the HTML5 spec, mark up navigation as a list inside a `nav` element for better semantics and accessibility.

see:

  * http://www.w3.org/html/wg/drafts/html/master/common-idioms.html#rel-up
  * http://lists.w3.org/Archives/Public/public-html/2013Nov/thread.html#msg6
  * https://dl.dropboxusercontent.com/u/377471/breadcrumb.html

for discussion and background.