Move to using NSIndexPaths in delegate methods

[SteveFortune]

NSIndexPath is a much cleaner* way of providing the metadata around the items that are being drag / dropped. If we are drag/dropping between UI[Collection | Table]Views, at the moment users have to implement boilerplate code to map a given point to an index path in the drag data source. It makes sense to do the following:

• Add indexPathForItemAtPoint to I3Collection
• Change itemAtPoint to itemAtIndexPath in I3Collection
• Delegate the responsibility of mapping index paths to actual item views, out to the collection implementation
• Document and provide use cases for how users might implement an NSIndexPath category for use with their own I3Collection implementations. For example, a UIView that implements I3Collection may extend NSIndexPath to provide metadata about the position of its subviews.

Refactor the coordinator and other related classes to deal only in NSIndexPaths. Now we can adequately encapsulate the responsibility of mapping a given point to an associated data index to the collection.

*its cleaner in my opinion, because CGPoints are meant to specify exactly where the item is on the screen. NSIndexPath provides data about 'which items belong where' in a more abstract data-oriented way