Implement socket_base_t::get_credential member function

The get_credential () member function returns
credential for the last peer we received message for.
The idea is that this function is used to implement user-level API.

src/pipe.hpp

@@ -78,6 +78,8 @@ namespace zmq
         void set_identity (const blob_t &identity_);
         blob_t get_identity ();
 
+        blob_t get_credential () const;
+
         //  Returns true if there is at least one message to read in the pipe.
         bool check_read ();
 
@@ -198,6 +200,9 @@ namespace zmq
         //  Identity of the writer. Used uniquely by the reader side.
         blob_t identity;
 
+        //  Pipe's credential.
+        blob_t credential;
+
         //  Returns true if the message is delimiter; false otherwise.
         static bool is_delimiter (const msg_t &msg_);