message: add attachment_upload to facebook_botkit.api
diff: @@ -610,9 +610,12 @@ function Facebookbot(configuration) {
  }
  };
  + var attachment_upload_api = {};
  +
  facebook_botkit.api = {
  'messenger_profile': messenger_profile_api,
  - 'thread_settings': messenger_profile_api
  + 'thread_settings': messenger_profile_api,
  + 'attachment_upload': attachment_upload_api
  };
  // Verifies the SHA1 signature of the raw request payload before bodyParser parses it
  