added accessibility enhancements for details element
fixed js to detect if open close state
swapped append and attr method calls, removed aria-expanded attr on panel
Update details/summary polyfill

- Set aria-controls, aria-expanded and aria-hidden attributes
- Check to see if details has open attribute and update aria attributes
- Add open attribute to test at /examples/details-summary
Set and update aria-hidden attribute
Remove console.log
Improve readability and adjust whitespace

Update check to see if detailsAttr is undefined
- Improve readability, as typeof undefined will always result in the
string 'undefined'

- Adjust whitespace
Merge branch with master.
Merge pull request #28 from alphagov/aria-for-details-summary-test

ARIA attributes for details summary polyfill