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/ndn-forwarding-strategy.h b/model/fw/ndn-forwarding-strategy.h
index d3b5c37..b838565 100644
--- a/model/fw/ndn-forwarding-strategy.h
+++ b/model/fw/ndn-forwarding-strategy.h
@@ -36,6 +36,7 @@
 namespace pit { class Entry; }
 class FibFaceMetric;
 class Fib;
+namespace fib { class Entry; }
 class ContentStore;
 
 /**
@@ -109,6 +110,20 @@
    */
   virtual void
   RemoveFace (Ptr<Face> face);
+
+  /**
+   * @brief Event fired every time a FIB entry is added to FIB
+   * @param fibEntry FIB entry that was added
+   */
+  virtual void
+  DidAddFibEntry (Ptr<fib::Entry> fibEntry);
+
+  /**
+   * @brief Fired just before FIB entry will be removed from FIB
+   * @param fibEntry FIB entry that will be removed
+   */
+  virtual void
+  WillRemoveFibEntry (Ptr<fib::Entry> fibEntry);
   
 protected:
   /**