model: Implementing two more events for forwarding strategy: DidAddFibEntry and WillRemoveFibEntry

Currently, only DidAddFibEntry method is used and only in PerFibLimits
strategy.  Change logic in global routing helper: set per FIB limits only if
forwarding strategy has created a corresponding Limit object.
diff --git a/model/fw/per-fib-limits.h b/model/fw/per-fib-limits.h
index 3632a87..47f9ee6 100644
--- a/model/fw/per-fib-limits.h
+++ b/model/fw/per-fib-limits.h
@@ -80,6 +80,19 @@
         exit (1);
       }
   }
+
+  /// \copydoc ForwardingStrategy::DidAddFibEntry
+  virtual void
+  DidAddFibEntry (Ptr<fib::Entry> fibEntry)
+  {
+    ObjectFactory factory;
+    factory.SetTypeId (fibEntry->m_faces.begin ()->m_face->GetObject<Limits> ()->GetInstanceTypeId ());
+    
+    Ptr<Limits> limits = factory.template Create<Limits> ();
+    fibEntry->AggregateObject (limits);
+
+    super::DidAddFibEntry (fibEntry);
+  }
   
 protected:
   /// \copydoc ForwardingStrategy::CanSendOutInterest