Drag delay feature

I am building on top of this directive and thought this might be a useful feature to add.
I created a ```data-drag-delay``` attribute. Using angular's ```$timeout``` to delay  the ```dragStartEvent```. This allows  for nodes to be clicked on or double clicked without creating buttons that use ```data-nodrag```. It is as simple as adding data-drag-delay to the ```ui-tree``` element and type how many milliseconds they want the user to hold the mouse down.