model: Major API changes

Interest and ContentObject are no longer derived from Header class.
Instead, they are encapsulating payload and, optionally, wire-formatted
Packet object.

Refs #1005 (http://redmine.named-data.net/)
diff --git a/model/cs/content-store-with-freshness.h b/model/cs/content-store-with-freshness.h
index 81f8856..3ba871c 100644
--- a/model/cs/content-store-with-freshness.h
+++ b/model/cs/content-store-with-freshness.h
@@ -90,9 +90,9 @@
 
 template<class Policy>
 inline bool
-ContentStoreWithFreshness< Policy >::Add (Ptr<const ContentObject> header, Ptr<const Packet> packet)
+ContentStoreWithFreshness< Policy >::Add (Ptr<const ContentObject> data)
 {
-  bool ok = super::Add (header, packet);
+  bool ok = super::Add (data);
   if (!ok) return false;
 
   NS_LOG_DEBUG (header->GetName () << " added to cache");