fixes
added a callback function "onReposition". also some bug fixes
minor fixes and updates to the HTML demo page
Correct a confusing variable name.
Add an 'onReposition' callback.

To be invoked with the floated element when the element is repositioned.
Allow elements to be stuck to target elements rather their parents' visible bounds.
Always reposition the floated element when `update` is called.

This is useful when tracking a target element, to 'resync' the floated element and target
elements in cases where the target element may have moved for other reasons than
the scroll area scrolling or the window resizing.