Define i_properties interface

- copy and move message operations are updated to maintain proper
  reference count of properties object
- zmq_msg_gets updated to use i_properties interface to fetch property
  value
- setter/getter added to msg_t class

src/msg.hpp

@@ -25,6 +25,7 @@
 
 #include "config.hpp"
 #include "atomic_counter.hpp"
+#include "i_properties.hpp"
 
 //  Signature for free function to deallocate the message content.
 //  Note that it has to be declared as "C" so that it is the same as
@@ -70,6 +71,8 @@ namespace zmq
         void reset_flags (unsigned char flags_);
         int64_t fd ();
         void set_fd (int64_t fd_);
+        i_properties *properties () const;
+        void set_properties (i_properties *properties_);
         bool is_identity () const;
         bool is_credential () const;
         bool is_delimiter () const;
@@ -86,8 +89,6 @@ namespace zmq
 
     private:
 
-        class i_properties;
-
         //  Size in bytes of the largest message that is still copied around
         //  rather than being reference-counted.
         enum { msg_t_size = 48 };