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-cache.hpp b/src/security/certificate-cache.hpp
index 42f3b88..fa6cb79 100644
--- a/src/security/certificate-cache.hpp
+++ b/src/security/certificate-cache.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2014 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).
  *
@@ -25,9 +25,10 @@
 #define NDN_SECURITY_CERTIFICATE_CACHE_HPP
 
 #include "../name.hpp"
-#include "identity-certificate.hpp"
+#include "v1/identity-certificate.hpp"
 
 namespace ndn {
+namespace security {
 
 /**
  * @brief Interface for the cache of validated certificates
@@ -41,9 +42,9 @@
   }
 
   virtual void
-  insertCertificate(shared_ptr<const IdentityCertificate> certificate) = 0;
+  insertCertificate(shared_ptr<const v1::IdentityCertificate> certificate) = 0;
 
-  virtual shared_ptr<const IdentityCertificate>
+  virtual shared_ptr<const v1::IdentityCertificate>
   getCertificate(const Name& certificateNameWithoutVersion) = 0;
 
   virtual void
@@ -59,6 +60,10 @@
   }
 };
 
+} // namespace security
+
+using security::CertificateCache;
+
 } // namespace ndn
 
 #endif // NDN_SECURITY_CERTIFICATE_CACHE_HPP