all: InterestHeader and ContentObjectHeader refactoring to Interest and ContentObject

refs #29
diff --git a/model/ndn-l3-protocol.h b/model/ndn-l3-protocol.h
index 40278d6..36330cb 100644
--- a/model/ndn-l3-protocol.h
+++ b/model/ndn-l3-protocol.h
@@ -39,8 +39,8 @@
 
 class Face;
 class ForwardingStrategy;
-class InterestHeader;
-class ContentObjectHeader;
+class Interest;
+class ContentObject;
 
 /**
  * \defgroup ndn ndnSIM: NDN simulation module