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-face.cc b/model/ccnx-face.cc
index e19dc57..3250355 100644
--- a/model/ccnx-face.cc
+++ b/model/ccnx-face.cc
@@ -34,6 +34,7 @@
 CcnxFace::GetTypeId (void)
 {
   static TypeId tid = TypeId ("ns3::CcnxFace")
+    .SetGroupName ("Ccnx")
     .SetParent<Object> ()
   ;
   return tid;