operator void* () added to context_t.

Makes it possible to share the context from C++ to C.
Signed-off-by: Thijs Terlouw <thijsterlouw@gmail.com>

include/zmq.hpp

@@ -184,6 +184,14 @@ namespace zmq
             assert (rc == 0);
         }
 
+        //  Be careful with this, it's probably only useful for
+        //  using the C api together with an existing C++ api.
+        //  Normally you should never need to use this.
+        inline operator void* ()
+        {
+            return ptr;
+        }
+        
     private:
 
         void *ptr;