provide newIndex value in onDrop event

[kennyrkun]

Hi there,

This is a really simple patch that solves a small problem I was having with the library.

I check if (event.oldIndex == event.newIndex && event.to == event.from) to determine if the item was actually moved, or just picked up and put down. The event always provided null for newIndex, so it was impossible to tell. Using the newIndex variable provided just above where the event is fired allowed me use this logic.

Previously, null was always provided for this event. I do not know why, or if it was necessary, but in my use case this fixed the problem and I haven't noticed any other issues. The usage guide in the readme says that this value should include the newIndex of the item, so I think that always providing null was unintended.