pawn movement added (no killing though)

getX(), getY(), getXDifference(), getYDifference(), canMove() functions
added.
Found out that x is horizontal and y is vertical.
Pawns can kill by moving onto the enemy.
Pawns are now fully implemented

Moving, killing, "en passant" trick, becoming queen(not fully
implemented, because there is no queen implemented yet, but last cell
detection is implemented already)