Implementing content store using Boost.MultiIndex container

Adding .SetGroupName("Ccnx") call to Ccnx-related objects (probably
missed some).  This call is advisory, but it is nice to give users
information about Ccnx-related objects (this information is accessible
via --PrintGroup=Ccnx command line argument)
diff --git a/model/ccnx-content-object-header.h b/model/ccnx-content-object-header.h
index c26052b..97a8d3d 100644
--- a/model/ccnx-content-object-header.h
+++ b/model/ccnx-content-object-header.h
@@ -24,6 +24,7 @@
 
 #include "ns3/integer.h"
 #include "ns3/header.h"
+#include "ns3/trailer.h"
 
 #include <string>
 #include <vector>
@@ -101,7 +102,7 @@
  * ContentObjectTail should always be 2 bytes, representing two closing tags:
  * "</Content><ContentObject>"
  */
-class CcnxContentObjectTail : public Header
+class CcnxContentObjectTail : public Trailer
 {
 public:
   CcnxContentObjectTail ();