574 adding keyboard accessibility  to the facet expanders

Bug #574 :

Adding a tab index and a button role to the h3 that constitutes the expandable facet header to enable keyboard navigation. Using the role button rather than link because the W3C recommends that if it doesn't change focus or page -- http://www.w3.org/TR/wai-aria/roles#link

Adding a keypress handler for pressing "enter" on that header, as well as a click handler.