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/ndn-content-store.cc b/model/cs/ndn-content-store.cc
index c99d7ed..cfe362e 100644
--- a/model/cs/ndn-content-store.cc
+++ b/model/cs/ndn-content-store.cc
@@ -34,7 +34,7 @@
 
 NS_OBJECT_ENSURE_REGISTERED (ContentStore);
 
-TypeId 
+TypeId
 ContentStore::GetTypeId (void)
 {
   static TypeId tid = TypeId ("ns3::ndn::ContentStore")
@@ -60,24 +60,12 @@
 
 //////////////////////////////////////////////////////////////////////
 
-Entry::Entry (Ptr<ContentStore> cs, Ptr<const ContentObject> header, Ptr<const Packet> packet)
+Entry::Entry (Ptr<ContentStore> cs, Ptr<const ContentObject> data)
   : m_cs (cs)
-  , m_header (header)
-  , m_packet (packet->Copy ())
+  , m_data (data)
 {
 }
 
-Ptr<Packet>
-Entry::GetFullyFormedNdnPacket () const
-{
-  static ContentObjectTail tail; ///< \internal for optimization purposes
-
-  Ptr<Packet> packet = m_packet->Copy ();
-  packet->AddHeader (*m_header);
-  packet->AddTrailer (tail);
-  return packet;
-}
-
 const Name&
 Entry::GetName () const
 {
@@ -85,15 +73,9 @@
 }
 
 Ptr<const ContentObject>
-Entry::GetHeader () const
+Entry::GetData () const
 {
-  return m_header;
-}
-
-Ptr<const Packet>
-Entry::GetPacket () const
-{
-  return m_packet;
+  return m_data;
 }
 
 Ptr<ContentStore>