added Callback pattern

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

@@ -0,0 +1,13 @@
+package com.iluwatar;
+
+/**
+ * Implementation of task that need to be executed
+ */
+public class SimpleTask extends Task {
+
+	@Override
+	public void execute() {
+		System.out.println("Perform some important activity.");
+	}
+
+}