model: Adding "new" content store with ability to track lifetime of entries

Available options for ndnStackHelper.SetContentStore:
- ns3::ndn::cs::Stats::Lru
- ns3::ndn::cs::Stats::Random
- ns3::ndn::cs::Stats::Fifo

Tracked events include 0-lifetime entries, i.e., when entries are not stored at all.

Config::ConnectWithoutContext ("/NodeList/1/$ns3::ndn::cs::Stats::Random/WillRemoveEntry", MakeCallback (CacheEntryRemoved));

Prototype for the callback is: void (*)(Ptr<const ndn::cs::Entry> entry, Time lifetime)
diff --git a/model/fib/ndn-fib-entry.h b/model/fib/ndn-fib-entry.h
index a98dc66..214b0eb 100644
--- a/model/fib/ndn-fib-entry.h
+++ b/model/fib/ndn-fib-entry.h
@@ -171,6 +171,9 @@
 class Entry : public Object
 {
 public:
+  typedef Entry base_type;
+  
+public:
   class NoFaces {}; ///< @brief Exception class for the case when FIB entry is not found
   
   /**