New demo application: nqueens

This is a new demo for the nqueens based on the X10 version.
For instance, what you could do right now is *not* make the new copy on line 123 here, and instead have nqSearch look like:

~~~ cpp
void nqSearch(GlobalAddress<Board> prevBoard, int i) {
  Board * myBoard = new Board(prevBoard, i);
  auto g_prevBoard = make_global(myBoard);
  for (int j =0; j<n; j++) {
    if (myBoard->isSafe(j)) {
      spawn<unbound, &gce>([=]{ nqSearch(g_prevBoard, j); });
    }
  }
}
~~~