refactored pawn movement + implemented knight movement

Board.java

@@ -26,6 +26,10 @@ public class Board {
   }
 
   public boolean isOpen(Position pos) {
-    return this.board[pos.x][pos.y] == null;
+    return this.getPiece(pos) == null;
+  }
+
+  public Piece getPiece(Position pos) {
+    return this.board[pos.x][pos.y];
   }
 }