security: Update Signature abstraction to match 0.1.1 spec

This commit removes intermediate SignatureWithPublicKey data
structure.  All uses of such data structure should be replaced with just
Signature and check that KeyLocator is present (hasKeyLocator()).

Change-Id: I1157e05c6a717ea28df91c4bfd76c7145b520a15
Refs: #1750
diff --git a/src/security/digest-sha256.hpp b/src/security/digest-sha256.hpp
index 6221ee6..d3a15de 100644
--- a/src/security/digest-sha256.hpp
+++ b/src/security/digest-sha256.hpp
@@ -43,21 +43,11 @@
     }
   };
 
-  DigestSha256()
-  {
-    m_info = Block(Tlv::SignatureInfo);
-
-    m_type = Tlv::DigestSha256;
-    m_info.push_back(nonNegativeIntegerBlock(Tlv::SignatureType, Tlv::DigestSha256));
-  }
+  DigestSha256();
 
   explicit
-  DigestSha256(const Signature& signature)
-    : Signature(signature)
-  {
-    if (getType() != Tlv::DigestSha256)
-      throw Error("Incorrect signature type");
-  }
+  DigestSha256(const Signature& signature);
+
 };
 
 } // namespace ndn