New demo application: nqueens

This is a new demo for the nqueens based on the X10 version.
That's what I thought. I did implemented it and apparently it is faster but the main issue is: when is it safe to delete `myBoard` (i.e., you cannot just delete `prevBoard` because it might be in use by other tasks) ? Maybe a smart pointer could help?

There are other minor issues such as the previous board having one less column and, if you create your local board using `prevBoard.size+1`, `isSafe()` needs to be changed to take that into account which, in turn, makes the code less intuitive. But performance-wise, it sure is better.