Make RKMappingOperation be a straight NSObject subclass instead of NSOperation; we were really only using the cancellation methods from the superclas (and -start).  There is actually a 10% penalty on the entire mapping process just for having NSOperation as a superclass for some reason, at least when using a lot of nested operations.
Fix compile warning in unit tests after this change.
Add documentation for public methods, and change the cancelled property to match the iOS8/10.10 NSOperation API (and to be a normal property).
Merge pull request #2071 from carllindberg/2065-MappingOperationSuperclass

Make RKMappingOperation be an NSObject subclass