added visitor pattern sample

visitor/src/main/java/com/iluwatar/Sergeant.java

@@ -0,0 +1,19 @@
+package com.iluwatar;
+
+public class Sergeant extends Unit {
+
+	public Sergeant(Unit ... children) {
+		super(children);
+	}
+
+	@Override
+	public void accept(UnitVisitor visitor) {
+		visitor.visitSergeant(this);
+		super.accept(visitor);
+	}
+	
+	@Override
+	public String toString() {
+		return "sergeant";
+	}
+}