Add enumerateKeysAndObjectsWithIndexUsingBlock method

Hi, this is a small Pull Request to add an enumeration method using block much like the [already existing one](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSDictionary_Class/Reference/Reference.html#//apple_ref/occ/instm/NSDictionary/enumerateKeysAndObjectsUsingBlock:) but with an extra argument, the index.