Aligned ARIA attributes with jQuery UI Tabs

This came up from #1018.  It matches the keyboard behaviour and aria-* attributes with [jQuery UI tabs](http://jqueryui.com/tabs/).

One exception is that in jQuery tabs, the `<li>` has role=tab.  The tabbed interface has this on the `<a>` nested in the `<li>`.  I can make this change, but wasn't sure if there was value since the aria attributes are the same, just on a different element.