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/ndn-l3-protocol.h b/model/ndn-l3-protocol.h
index 292de13..406e194 100644
--- a/model/ndn-l3-protocol.h
+++ b/model/ndn-l3-protocol.h
@@ -39,16 +39,11 @@
 
 class Face;
 class ForwardingStrategy;
-class Interest;
-class ContentObject;
-
-typedef Interest InterestHeader;
-typedef ContentObject ContentObjectHeader;
 
 /**
  * \defgroup ndn ndnSIM: NDN simulation module
  *
- * This is a simplified modular implementation of NDN protocol
+ * This is a modular implementation of NDN protocol for NS-3
  */
 /**
  * \ingroup ndn