getting wrong mouse position if page is scrolled while dragging

[hangy233]

This can be reproduced in the example, resize your browser window and make it shorter than the tree.
if you scroll the window when you are dragging a node. The node will go to a wrong position.
Solutions could be stop drag and drop the node at where it is when start a scroll
or count the scrollTop value in the drag handler.

[hangy233]

made a pull request #37