Create a binding for zmq_msg_gets.

Additionally, I've changed some tabs to spaces and remove trailing
spaces.

zmq.hpp

@@ -372,6 +372,14 @@ namespace zmq
             return a == b;
         }
 
+        inline const char* gets(const char *property_) const
+        {
+            const char* value = zmq_msg_gets (&msg, property);
+            if (value == NULL)
+                throw error_t ();
+            return value;
+        }
+
     private:
         //  The underlying message
         zmq_msg_t msg;