Add unit test to show that the callback method is called.

Author: Richard Jones

callback/src/main/java/com/iluwatar/callback/SimpleTask.java

@@ -9,7 +9,7 @@ public class SimpleTask extends Task {
 
 	@Override
 	public void execute() {
-		System.out.println("Perform some important activity.");
+		System.out.println("Perform some important activity and after call the callback method.");
 	}
 
 }