support optional 'container'

[sporritt]

the container being the parent element of the draggables.

• if an element is dragged outside of the container a dragExit event should be fired, and a class should be added to the container (and to the 'outside' element)
• if an element is dragged outside of the container and dropped, one of three things should happen:
  • if the element is dropped on some other katavorio container, control for the element should be passed to the katavorio instance controlling the given container. this means re-registering all Drags and Drops for the dragged element with the other Katavorio instance.
  • if 'revert' flag (or similar name) is true, the element should revert to its original location before the drag
  • if 'revert' is false and the element is not dropped on some other katavorio container, it should be left where it is.

Some other, slightly more hazy, requirements right now...the above setup works only if the user has set overflow:visible to the container. It is not likely that this will always be the case. In the event that overflow is not set to visible, one of two things could happen:

• we could clone the current node, append it to the body, and move on. in this case if the element was not dropped on some other katavorio we would have to act as if revert was set to true.
• we could ask the user for a proxy node to use while dragging outside of the container. The same rule about reverting position would apply.