add events.drag plugin
Revert 'add events.drag plugin'
fix 'mousedown' in _downHandler

'mouseup' was dispatched instead of 'mousedown' in _downHandler
bind 'mousedown'
Revert the deletion of examples/drag-nodes.html
Merge remote-tracking branch 'origin/master' into fix-mouse-down
Merge remote branch 'origin/pr/305'