Crash while moving Cells - Tested on iOS 5.1

I was trying to create a Drag & Drop CollectionView and it was working fine in iOS 6.
But using PSTCollectionView only works moving cells from the first section.

```objectivec
NSUInteger originalIndex = [newModel[updateItem.indexPathBeforeUpdate.section] indexOfObject:@(updateItem.indexPathBeforeUpdate.item)];
id object = newModel[updateItem.indexPathBeforeUpdate.section][originalIndex];
```
`originalIndex` was `NSNotFound` so it crash.

because it was only looking for objects only in the first section, so by doing this:
```objectivec
id object = @([oldCollectionViewData globalIndexForItemAtIndexPath:updateItem.indexPathBeforeUpdate]);
```

is using the global index who was stored in the `newModel` array

Let me know if I'm wrong.

Thanks Guys! PSTCollectionView it's helping me a lot.