Make tooltips accessible using WAI-ARIA.

To do so we have to make to modifications:
-Setting role="tooltip" on the element that is the tooltip
-Setting aria-describedby on the element that the tooltip is about

The latter requires the ID of the tooltip. I've written a small function - inspired by MooTools - to add a unique ID to any element if it doesn't have one. I chose a different name than the one jQuery UI has so there's no conflict.

The setUniqueId methods lacks documentation and unit tests. I'll happily write the former if this gets a thumb up, about the latter I have to do some reading up first.

I chose to replace the tooltip class in the LESS with the role attribute since they express the same thing and they seemed like duplicate information.