Implemented perft

Patch from Joona with extension to benchmark and inclusion
of Depth(0) moves generation by me.

Note that to test also qsearch and in particulary checks
generations a change in the end condition is needed.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/search.h

@@ -69,6 +69,7 @@ extern void stop_threads();
 extern bool think(const Position &pos, bool infinite, bool ponder, int side_to_move,
                   int time[], int increment[], int movesToGo, int maxDepth,
                   int maxNodes, int maxTime, Move searchMoves[]);
+extern int perft(Position &pos, Depth depth);
 extern int64_t nodes_searched();