added accessibility enhancements for details element

Making the details element more accessible by adding role="button" attribute, aria-expanded and screenreader help text. This informs the user that the interaction is a button and it's reveal/hide states. 
I don't think we need an `aria-expanded` on the panel. The information that it's expanded or not is communicated by it being set on the controlling element which we're already doing.

Some evidence in the spec http://www.w3.org/TR/wai-aria/states_and_properties#aria-expanded (but a better written explanation is http://www.marcozehe.de/2010/02/10/easy-aria-tip-5-aria-expanded-and-aria-controls/).