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