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)
regarding the callout, I did this for consistency with the "Make modals accessible" callout in the modals documentation, which is then pulled together on this same docs page http://getbootstrap.com/javascript/#modals-examples

agree that the "Via data attributes" section wasn't the most obvious fit - I've moved it under the "Examples" section, just before "Usage", again to keep it consistent with how it's done on the modals part of the docs. Yes, "Examples" is an `h2` and my "Make expand/collapse controls accessible" heading is an `h4`, but again this is for consistency, and has no real negative impact (even in terms of accessibility/screen reader users navigating the docs).