New demo application: nqueens

This is a new demo for the nqueens based on the X10 version.
If at the point I'm calling `isSafe()` I had the local copy of the previous board it surely would work. However, that would require a call to something like `board->increaseSize(1)` afterwards to account for the extra column the next tasks will use. Now, this increase cannot be done easily inside the main `for()` because it will add an extra column per spawned task.

So what I am doing is creating a local board with an extra column, which forces me to change the `isSafe()` method not to look in this extra column.