model: New content store variations: support for content freshness

The following content store realizations now fully support freshness set
in ContentObjects:
- ns3::ndn::cs::Freshness::Lru
- ns3::ndn::cs::Freshness::Random
- ns3::ndn::cs::Freshness::Fifo

Example ndn-simple-with-content-freshness demonstrates basics of new
content stores.
diff --git a/examples/wscript b/examples/wscript
index cbd026c..13667ca 100644
--- a/examples/wscript
+++ b/examples/wscript
@@ -7,9 +7,14 @@
     obj = bld.create_ns3_program('ndn-grid', ['ndnSIM', 'point-to-point-layout'])
     obj.source = 'ndn-grid.cc'
     
+    obj = bld.create_ns3_program('ndn-simple-with-content-freshness', ['ndnSIM'])
+    obj.source = ['ndn-simple-with-content-freshness.cc',
+                  'custom-apps/dumb-requester.cc']
+
     obj = bld.create_ns3_program('ndn-simple-with-custom-app', ['ndnSIM'])
     obj.source = ['ndn-simple-with-custom-app.cc',
-                  'custom-apps/custom-app.cc']
+                  'custom-apps/custom-app.cc',
+                  'custom-apps/hijacker.cc']
 
     if 'topology' in bld.env['NDN_plugins']:
         obj = bld.create_ns3_program('ndn-grid-topo-plugin', ['ndnSIM'])