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)
replacing my if/else with the the two liner of var and toggling makes some tests fail, strangely...although functionally it seems to be equivalent. will dig into it again later.