Implement copy constructor for class Monitor

To support the instance copy of Monitor like the following:
    AMQP::Monitor monitor(consumer);
    consumer->bindExchange(exchange, AMQP::direct).onSuccess([&, monitor](){
        // do something when the consumer is ready!
        if(monitor.valid())
            consumer->doSomething();
    });

include/monitor.h

@@ -57,6 +57,16 @@ public:
         _watchable->add(this);
     }
 
+    /**
+     *  Copy constructor
+     *  @param  monitor
+     */
+    Monitor(const Monitor &monitor) : _watchable(monitor._watchable)
+    {
+        // register with the watchable
+        _watchable->add(this);
+    }
+
     /**
      *  Destructor
      */