[NSPointerArray strongObjectsPointerArray] not available in 10.7

I was testing out some code with PromiseKit in OS X 10.7 and ran into an issue with `strongObjectsPointerArray` which isn’t available until 10.8. As far as I can tell, pre 10.8 should use `pointerArrayWithStrongObjects` instead (which became deprecated in 10.8).

I added a switch to change the `NSPointerArray` creation method depending on `[NSPointerArray respondsToSelector:@selector(strongObjectsPointerArray)]`.