code: func (m *Messenger) Handler(rw http.ResponseWriter, req *http.Request) {
  	if req.Method == "GET" {
  		query := req.URL.Query()
  		if query.Get("hub.verify_token") != m.VerifyToken {
  			rw.WriteHeader(http.StatusUnauthorized)
  			return
  		}
  		rw.WriteHeader(http.StatusOK)
  		rw.Write([]byte(query.Get("hub.challenge")))
  	} else if req.Method == "POST" {
  		m.handlePOST(rw, req)
  	} else {
  		rw.WriteHeader(http.StatusMethodNotAllowed)
  	}
  }
docs: // Handler is the main HTTP handler for the Messenger service.
  // It MUST be attached to some web server in order to receive messages