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-out-face-limits.h b/model/fw/per-out-face-limits.h
index 534768b..2c4a935 100644
--- a/model/fw/per-out-face-limits.h
+++ b/model/fw/per-out-face-limits.h
@@ -133,10 +133,10 @@
 template<class Parent>
 bool
 PerOutFaceLimits<Parent>::CanSendOutInterest (Ptr<Face> inFace,
-                                          Ptr<Face> outFace,
-                                          Ptr<const InterestHeader> header,
-                                          Ptr<const Packet> origPacket,
-                                          Ptr<pit::Entry> pitEntry)
+                                              Ptr<Face> outFace,
+                                              Ptr<const InterestHeader> header,
+                                              Ptr<const Packet> origPacket,
+                                              Ptr<pit::Entry> pitEntry)
 {
   NS_LOG_FUNCTION (this << pitEntry->GetPrefix ());