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/tools/ndnsec/sign-req.hpp b/tools/ndnsec/sign-req.hpp
index d3ec904..bb8906e 100644
--- a/tools/ndnsec/sign-req.hpp
+++ b/tools/ndnsec/sign-req.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).
  *
@@ -30,6 +30,7 @@
 ndnsec_sign_req(int argc, char** argv)
 {
   using namespace ndn;
+  using namespace ndn::security;
   namespace po = boost::program_options;
 
   std::string name;
@@ -75,7 +76,7 @@
   if (vm.count("key") != 0)
     isKeyName = true;
 
-  shared_ptr<IdentityCertificate> selfSignCert;
+  shared_ptr<v1::IdentityCertificate> selfSignCert;
 
   KeyChain keyChain;