added method "integers"

useful macro to make index list :)
I'm an advocate of self-describing method signatures and I don't think `- (NSArray *)integers` really describe the purpose of this method (aside: it's _not_ a macro). 

`NSArray *indices = [@9.80665 integers];` doesn't make sense to me. What is the use-case here? Would this be better served as a function (e.g. `NSArray* OSIndexArrayWithUpperBound(NSUInteger upperBound)`) rather than a method?