Refactor KFAttribute so it becomes a protocol/multiple objects

[kylef]

Multiple protocols so you can have type checking on different types of attributes, to-many, to-one, etc.

@protocol KFToManyAttribute

- (NSPredicate *)in:(id<NSFastEnumeration>)objects;

@end

[kylef]

Swift version uses Generics and provides this kind of thing.