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/util/string-helper.cpp b/src/util/string-helper.cpp
index 16064b0..0f54e14 100644
--- a/src/util/string-helper.cpp
+++ b/src/util/string-helper.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).
  *
@@ -21,7 +21,7 @@
 
 #include "string-helper.hpp"
 #include "../encoding/buffer-stream.hpp"
-#include "../security/cryptopp.hpp"
+#include "../security/v1/cryptopp.hpp"
 
 #include <sstream>
 #include <iomanip>