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.cc b/model/ccnx.cc
index a940c36..0fe4b31 100644
--- a/model/ccnx.cc
+++ b/model/ccnx.cc
@@ -32,6 +32,7 @@
 Ccnx::GetTypeId (void)
 {
   static TypeId tid = TypeId ("ns3::Ccnx")
+    .SetGroupName ("Ccnx")
     .SetParent<Object> ()
   ;
   return tid;