security: Signature interface change and initial fixes for OSX private key store

Change-Id: Ic6d074e99346c4bf9d7b2c20c961dd01d770636d
diff --git a/include/ndn-cpp/security/identity/memory-private-key-storage.hpp b/include/ndn-cpp/security/identity/memory-private-key-storage.hpp
index f267583..ee4f706 100644
--- a/include/ndn-cpp/security/identity/memory-private-key-storage.hpp
+++ b/include/ndn-cpp/security/identity/memory-private-key-storage.hpp
@@ -67,10 +67,10 @@
    * @return The signature, or a null pointer if signing fails.
    */  
   virtual Block 
-  sign(const uint8_t *data, size_t dataLength, const Signature &signature, const Name& keyName, DigestAlgorithm digestAlgorithm);
+  sign(const uint8_t *data, size_t dataLength, const Name& keyName, DigestAlgorithm digestAlgorithm);
 
-  virtual Block 
-  sign(const Data &data, const Signature &signature, const Name& keyName, DigestAlgorithm digestAlgorithm);
+  virtual void 
+  sign(Data &data, const Name& keyName, DigestAlgorithm digestAlgorithm);
   
   /**
    * Decrypt data.