func Twit(w http.ResponseWriter, r *http.Request) {
    defaultResponse := &SlashResponse{ResponseType: "ephemeral", Text: "success"}

    // prepare to make a slack delayed response
    responseURL := r.FormValue("response_url")
    go sendDelayResponse(responseURL, r)

    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(200)
    json.NewEncoder(w).Encode(defaultResponse)
}

func sendDelayResponse(url string, r *http.Request) {
    response := &SlashResponse{ResponseType: "in_channel", Text: twit.TweetTCL(r)}
    b, _ := json.Marshal(response)

    // send request to slack using given response_url
    ctx := appengine.NewContext(r)
    client := urlfetch.Client(ctx)
    req, _ := http.NewRequest("POST", url, bytes.NewBuffer(b))
    req.Header.Add("Content-Type", "application/json")
    resp, err := client.Do(req)
    defer resp.Body.Close()

    if err != nil {
        log.Println(err)
    } else {
        log.Println("success")
    }
}