allow non-dragable elements in dragable elements of the same itemNodeName

In my case, there can be deeply nested lists inside handle divs of
draggable list items ...and I don't want them to be draggable. All that
I want and expect to be draggable is defined via handleClass.
Merge pull request #5 from tobibeer/nonDraggableItemNodes

allow non-dragable elements in dragable elements of the same itemNodeNam...