Fixed undoMove method

[dawid-niedzwiecki]

(which worked only when moved pieces without capturing). There was this if statement:

    if (game.half_moves == 0) {
      return;
    }

which made it impossible to undo a move that was either a capturing move or pawn move (or both), since half_moves isn't the number of moves made. It's the number of moves counting towards the 50-move-draw rule. (Also the game.undo_move(); method is internally already checking if the game history is empty.)

It's a fix to issue #27