DER encoding: Added CertificateDataVisitor and use in Certificate::decode.
diff --git a/ndn-cpp/encoding/der/visitor/void-visitor.hpp b/ndn-cpp/encoding/der/visitor/void-visitor.hpp
index 7cd7999..e853ee6 100644
--- a/ndn-cpp/encoding/der/visitor/void-visitor.hpp
+++ b/ndn-cpp/encoding/der/visitor/void-visitor.hpp
@@ -18,15 +18,15 @@
 class VoidVisitor
 {
 public:
-  virtual void visit(DerBool&,             ndnboost::any) = 0;
-  virtual void visit(DerInteger&,          ndnboost::any) = 0;
-  virtual void visit(DerPrintableString&,  ndnboost::any) = 0;
-  virtual void visit(DerBitString&,        ndnboost::any) = 0;
-  virtual void visit(DerNull&,             ndnboost::any) = 0;
-  virtual void visit(DerOctetString&,      ndnboost::any) = 0;
-  virtual void visit(DerOid&,              ndnboost::any) = 0;
-  virtual void visit(DerSequence&,         ndnboost::any) = 0;
-  virtual void visit(DerGtime&,            ndnboost::any) = 0;
+  virtual void visit(DerBool&,             ndnboost::any);
+  virtual void visit(DerInteger&,          ndnboost::any);
+  virtual void visit(DerPrintableString&,  ndnboost::any);
+  virtual void visit(DerBitString&,        ndnboost::any);
+  virtual void visit(DerNull&,             ndnboost::any);
+  virtual void visit(DerOctetString&,      ndnboost::any);
+  virtual void visit(DerOid&,              ndnboost::any);
+  virtual void visit(DerSequence&,         ndnboost::any);
+  virtual void visit(DerGtime&,            ndnboost::any);
 };
 
 } // der