[WIP] Drag and drop support

[JoelStansbury]

I should have some more time to work on this in a couple weeks (after finals). The key findings are the need to add the dnd option to the plugins list and that the move event can be triggered on via move_node.jstree.

The current logic for updating children nodes and all that (327-353) fails to update other copies of the ipytree widget (due to the silent option) so that needs to be reworked.

Also, there should probably be an option for enabling and disabling dnd. I'm not sure how to do that yet.

[JoelStansbury]

I just found out that the duplication glitch shows up again when multi-select is enabled. Putting this back into WIP