Implementing base support for TCP-style window-based limiting on per-FIB-prefix and per-face granularity

Currently, limits are supported only by ndn::fw::Limits forwarding
strategy and only at the very basic (not fully tested) level.
diff --git a/model/ndn-face.h b/model/ndn-face.h
index 6ed40bc..d22e073 100644
--- a/model/ndn-face.h
+++ b/model/ndn-face.h
@@ -29,6 +29,7 @@
 #include "ns3/nstime.h"
 #include "ns3/type-id.h"
 #include "ns3/traced-callback.h"
+#include "ns3/ndn-limits.h"
 
 namespace ns3 {
 
@@ -89,7 +90,10 @@
   RegisterProtocolHandler (ProtocolHandler handler);
 
   /**
+   * @brief Get reference to Limits object
    */
+  inline Ptr<Limits>
+  GetLimits ();    
   
   /**
    * \brief Send packet on a face
@@ -217,6 +221,7 @@
   uint32_t m_id; ///< \brief id of the interface in Ndn stack (per-node uniqueness)
   uint32_t m_metric; ///< \brief metric of the face
 
+  Ptr<Limits> m_limits;
   // bool m_enableMetricTagging;
 
   TracedCallback<Ptr<const Packet> > m_txTrace;
@@ -250,6 +255,13 @@
   return !(*this == face);
 }
 
+inline Ptr<Limits>
+Face::GetLimits ()
+{
+  return m_limits;
+}
+
+
 } // namespace ndn
 } // namespace ns3