unregister event disconnect

https://github.com/ntoff/OctoPrint-Estop/issues/4#issuecomment-371146397

octoprint_estop/static/js/estop.js

@@ -55,6 +55,7 @@ $(function() {
                     OctoPrint.connection.disconnect();          //send a disconnect, maybe useful for breaking out of blocking commands.
 
                     self.onEventDisconnected = function () {     //wait until octoprint has disconnected
+                        self.onEventDisconnected = null;         //unregister event handler
                         OctoPrint.connection.connect();          //reconnect
                     }
                 }