New demo application: nqueens

This is a new demo for the nqueens based on the X10 version.
I guess I'm not sure, if you're making the local copy, why you'd have to change `isSafe()`. At the point that you're calling that, you'll have a board that's configured the same as before.

Your point about not knowing when it's safe to delete the previous board is valid. For now, we can just add a counter for each user and decrement it manually, deleting when it reaches 0 (basically just doing a bit more in your delegate on line 107). I agree, it would be cool to have a reference-counting smart pointer for cases like this. Perhaps that could be part of what you implement in the coming weeks!