ARIA attributes for details summary polyfill

- Update details/summary polyfill so the correct ARIA attributes are being set
  Ensure aria-expanded is set on the summary element
  Ensure aria-hidden is set on the toggled content

- Update test page: /examples/details-summary/

Doesn't look like the `hasAttribute` method is supported for IE7 and below: http://www.quirksmode.org/dom/core/#t125 so think we might need to use `getAttribute` instead: http://www.quirksmode.org/dom/core/#t123. There is some good guidance here: http://www.quirksmode.org/dom/core/#attributes.