helper+docs+examples: New helper to simplify link failing

Thanks to Saran Tarnoi for providing base implementation of the code
diff --git a/examples/ndn-simple-with-link-failure.cc b/examples/ndn-simple-with-link-failure.cc
new file mode 100644
index 0000000..161e735
--- /dev/null
+++ b/examples/ndn-simple-with-link-failure.cc
@@ -0,0 +1,104 @@
+/* -*-  Mode: C++; c-file-style: "gnu"; indent-tabs-mode:nil; -*- */
+/*
+ * Copyright (c) 2012 University of California, Los Angeles
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation;
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ * Author: Alexander Afanasyev <alexander.afanasyev@ucla.edu>
+ *       : Saran Tarnoi <saran.tarnoi@gmail.com>
+ */
+// ndn-simple-with-link-failure.cc
+#include "ns3/core-module.h"
+#include "ns3/network-module.h"
+#include "ns3/point-to-point-module.h"
+#include "ns3/ndnSIM-module.h"
+
+// for LinkStatusControl::FailLinks and LinkStatusControl::UpLinks
+#include "ns3/ndn-link-control-helper.h"
+
+using namespace ns3;
+
+/**
+ * This scenario simulates a very simple network topology:
+ *
+ *
+ *      +----------+     1Mbps      +--------+     1Mbps      +----------+
+ *      | consumer | <------------> | router | <------------> | producer |
+ *      +----------+         10ms   +--------+          10ms  +----------+
+ *
+ *
+ * Consumer requests data from producer with frequency 10 interests per second
+ * (interests contain constantly increasing sequence number).
+ *
+ * For every received interest, producer replies with a data packet, containing
+ * 1024 bytes of virtual payload.
+ *
+ * To run scenario and see what is happening, use the following command:
+ *
+ *     NS_LOG=ndn.Consumer:ndn.Producer ./waf --run=ndn-simple
+ */
+
+int 
+main (int argc, char *argv[])
+{
+  // setting default parameters for PointToPoint links and channels
+  Config::SetDefault ("ns3::PointToPointNetDevice::DataRate", StringValue ("1Mbps"));
+  Config::SetDefault ("ns3::PointToPointChannel::Delay", StringValue ("10ms"));
+  Config::SetDefault ("ns3::DropTailQueue::MaxPackets", StringValue ("20"));
+
+  // Read optional command-line parameters (e.g., enable visualizer with ./waf --run=<> --visualize
+  CommandLine cmd;
+  cmd.Parse (argc, argv);
+
+  // Creating nodes
+  NodeContainer nodes;
+  nodes.Create (3);
+
+  // Connecting nodes using two links
+  PointToPointHelper p2p;
+  p2p.Install (nodes.Get (0), nodes.Get (1));
+  p2p.Install (nodes.Get (1), nodes.Get (2));
+
+  // Install NDN stack on all nodes
+  ndn::StackHelper ndnHelper;
+  ndnHelper.SetDefaultRoutes (true);
+  ndnHelper.InstallAll ();
+
+  // Installing applications
+
+  // Consumer
+  ndn::AppHelper consumerHelper ("ns3::ndn::ConsumerCbr");
+  // Consumer will request /prefix/0, /prefix/1, ...
+  consumerHelper.SetPrefix ("/prefix");
+  consumerHelper.SetAttribute ("Frequency", StringValue ("10")); // 10 interests a second
+  consumerHelper.Install (nodes.Get (0)); // first node
+
+  // Producer
+  ndn::AppHelper producerHelper ("ns3::ndn::Producer");
+  // Producer will reply to all requests starting with /prefix
+  producerHelper.SetPrefix ("/prefix");
+  producerHelper.SetAttribute ("PayloadSize", StringValue("1024"));
+  producerHelper.Install (nodes.Get (2)); // last node
+
+  // The failure of the link connecting consumer and router will start from seconds 10.0 to 15.0
+  Simulator::Schedule (Seconds (10.0), ndn::LinkControlHelper::FailLink, nodes.Get (0), nodes.Get (1));
+  Simulator::Schedule (Seconds (15.0), ndn::LinkControlHelper::UpLink,   nodes.Get (0), nodes.Get (1));   
+  
+  Simulator::Stop (Seconds (20.0));
+
+  Simulator::Run ();
+  Simulator::Destroy ();
+
+  return 0;
+}