Progress-bar accessibility -- Iet ou/cr635/a11y progress

Hi John,

When you have a moment please review and pull. Relates to bug #635.

These fixes make the progress bar keyboard and screen reader accessible by,

* assigning the `slider` role;
* adding `tabindex=0` so keyboard users can navigate via TAB;
* Adding properties including "aria-valuetext" and `aria-now`;

Yours

Nick