Merge pull request #1 from brcosm/pullreq

Fixed a typo in ArtemisWorld
Changed 'NSObject*' types to id to reduce the amount of casting required when using the library.

Replaced explicit return types with 'instancetype' in class methods where a related result type is returned.

Replaced explicit types with 'self' (which refers to the class in a class method) in class methods that returned allocated instances of the same class.