examples: handle Nack in consumer

refs #3701
Change-Id: Ia6cfcefcf5a9541b09a50d1e5edaa55227908191
diff --git a/examples/consumer.cpp b/examples/consumer.cpp
index 4042565..0e0ff43 100644
--- a/examples/consumer.cpp
+++ b/examples/consumer.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2015 Regents of the University of California.
+ * Copyright (c) 2013-2016 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -42,6 +42,7 @@
 
     m_face.expressInterest(interest,
                            bind(&Consumer::onData, this,  _1, _2),
+                           bind(&Consumer::onNack, this, _1, _2),
                            bind(&Consumer::onTimeout, this, _1));
 
     std::cout << "Sending " << interest << std::endl;
@@ -58,6 +59,13 @@
   }
 
   void
+  onNack(const Interest& interest, const lp::Nack& nack)
+  {
+    std::cout << "received Nack with reason " << nack.getReason()
+              << " for interest " << interest << std::endl;
+  }
+
+  void
   onTimeout(const Interest& interest)
   {
     std::cout << "Timeout " << interest << std::endl;