Firefox and IE apply the focus pseudo class to anchors when clicked.
This prevents our toggle button returning to it's natural state after
it has been clicked. This change blurs the element on mouseup to ensure
that:

 1) The focus state is removed when clicked.
 2) The focus state remains when using the enter key to trigger the
    toggle (also triggers a click event). Otherwise the user would lose
    the focused UI element when navigating with a keyboard.