ARIA / screenreader support for collapse.js

Added handling of aria-expanded=true/false to collapse.js, updated
documentation to include advice on making expand/collapse controls
accessible, updated examples and javascript documentation to use
aria-expanded and aria-controls (when targetting single collapsible
element, using ID rather than class selector)
Please use [`equal`](http://api.qunitjs.com/equal/) for these kinds of assertions.