Add load/save functions

src/GomokuCell.java

@@ -151,9 +151,9 @@ public class GomokuCell{
             case Color.BLACK:
                 return "X";
             case Color.WHITE:
-                return "0";
+                return "O";
             default:
-                return "BUG";
+                throw new IllegalStateException("Unexpected value: " + this.getState());
         }
     }
 }