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

refs #29
diff --git a/model/ndn-app-face.h b/model/ndn-app-face.h
index 276fdfd..3a90f11 100644
--- a/model/ndn-app-face.h
+++ b/model/ndn-app-face.h
@@ -33,6 +33,10 @@
 
 class Interest;
 class ContentObject;
+
+typedef Interest InterestHeader;
+typedef ContentObject ContentObjectHeader;
+
 class App;
 
 /**