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.cpp b/src/security/certificate-container.cpp
index b18bbcb..a05dd52 100644
--- a/src/security/certificate-container.cpp
+++ b/src/security/certificate-container.cpp
@@ -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).
  *
@@ -32,7 +32,7 @@
 {
 }
 
-IdentityCertificate
+v1::IdentityCertificate
 CertificateContainer::const_iterator::operator*()
 {
   return m_impl->getCertificate(*m_it);