addObserver changed to use nil instead of self

MGSNObject.m

- (void)addObserver:(id)obj forSelector:(SEL)selector success:(SEL)success failure:(SEL)failure {
	[[NSNotificationCenter defaultCenter] addObserver:obj selector:success name:[self successNotificationNameForSelector:selector] object:nil];
	[[NSNotificationCenter defaultCenter] addObserver:obj selector:failure name:[self failureNotificationNameForSelector:selector] object:nil];
}