code: public ButtonTemplateBuilder addPostbackButton(String title, String payload) {
  		Button button = ButtonFactory.createPostbackButton(title, payload);
  		this.payload.addButton(button);
  		return this;
  	}
docs: Adds a button which sends a payload back when clicked to the current
  template. There can be at most 3 buttons.
  
  @param title
  the button label.
  @param payload
  the payload to send back when clicked.
  @return this builder.