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/key-gen.hpp b/tools/ndnsec/key-gen.hpp
index 16ce9c2..1384ac8 100644
--- a/tools/ndnsec/key-gen.hpp
+++ b/tools/ndnsec/key-gen.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).
  *
@@ -110,7 +110,7 @@
 
     keyChain.setDefaultKeyNameForIdentity(keyName);
 
-    shared_ptr<IdentityCertificate> identityCert = keyChain.selfSign(keyName);
+    shared_ptr<security::v1::IdentityCertificate> identityCert = keyChain.selfSign(keyName);
 
     if (isDefault)
       keyChain.setDefaultIdentity(Name(identityName));