* fixed an error in mindmap.js.
 * select a node when drag it.
 * ajusted code of droppable.js.
 * added pos option to droppable event.
 * maked empty node look more beautifull.
 * make node draggable.
 * can changed order and subside by dray node.
 * renamed 'shortcuts.js' to 'hotkeys.js'.
 * added hotkey to mindmap.
 * updated hotkeys handlers of mindmap.