security: Reorganizing source code to prepare for support of two version of NDN certificates

This commit also removes unused ndn_digestSha256 function and deprecates
crypto::sha256 in favor of crypto::computeSha256Digest in util/crypto.hpp.

Change-Id: I24ee50ff073a96b868633bdf2cfade412d3605f3
Refs: #3098
diff --git a/src/security/certificate-container.hpp b/src/security/certificate-container.hpp
index 1d7b6b4..f0cc408 100644
--- a/src/security/certificate-container.hpp
+++ b/src/security/certificate-container.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2015 Regents of the University of California.
+ * Copyright (c) 2013-2016 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -23,7 +23,7 @@
 #define NDN_SECURITY_CERTIFICATE_CONTAINER_HPP
 
 #include <set>
-#include "identity-certificate.hpp"
+#include "v1/identity-certificate.hpp"
 
 namespace ndn {
 namespace security {
@@ -40,7 +40,7 @@
     friend class CertificateContainer;
 
   public:
-    IdentityCertificate
+    v1::IdentityCertificate
     operator*();
 
     const_iterator&