added command sample

command/src/main/java/com/iluwatar/InvisibilitySpell.java

@@ -0,0 +1,28 @@
+package com.iluwatar;
+
+public class InvisibilitySpell extends Command {
+
+	private Target target;
+	
+	public InvisibilitySpell() {
+		target = null;
+	}
+	
+	@Override
+	public void execute(Target target) {
+		target.setVisibility(Visibility.INVISIBLE);
+		this.target = target;
+	}
+
+	@Override
+	public void undo() {
+		if (target != null) {
+			target.setVisibility(Visibility.VISIBLE);
+		}
+	}
+
+	@Override
+	public String toString() {
+		return "Invisibility spell";
+	}
+}