code: function renderToString(page) {
    var layout = null, child = null, props = {};
    while ((layout = page.type.layout || (page.defaultProps && page.defaultProps.layout))) {
      child = React.createElement(page, props, child);
      _.extend(props, page.defaultProps);
      React.renderToString(React.createElement(page, props, child));
      page = layout;
    }
    return React.renderToString(React.createElement(page, props, child));
  }
docs: Check if Page component has a layout property; and if yes, wrap the page
  into the specified layout, then render to a string.