1. CPSet and CPMutableSet are now abstract super classes that create appropriate concrete subclasses behind the scenes.
2. CPSet subclassers need only implement -count, -member:, and -objectEnumerator.
3. CPMutableSet subclassers need only implement -count, -member:, -objectEnumerator:, -addObject:, and removeObject:.
4. Added more CPSet tests.
5. Added _CPKeyValueCodingSet which subclasses CPSet for set returns in valueForKey:.
Reviewed by me.