all: Adding typedefs near forward declarations of Interest/ContentObject/Name

refs #29
diff --git a/model/ndn-l3-protocol.h b/model/ndn-l3-protocol.h
index 36330cb..2505a63 100644
--- a/model/ndn-l3-protocol.h
+++ b/model/ndn-l3-protocol.h
@@ -42,6 +42,9 @@
 class Interest;
 class ContentObject;
 
+typedef Interest InterestHeader;
+typedef ContentObject ContentObjectHeader;
+
 /**
  * \defgroup ndn ndnSIM: NDN simulation module
  *