title: Modal.propTypes - how to write definition with nested items
body: <p>I have this props and I like to write them into a Modal.propTypes Definition</p>
  
  <pre><code>'modalStyle': {
              'width': '800px',
              'height': '550px',
              'showOverlay': true,
              'showTitle': true,
              'title': 'my title'
          }
  </code></pre>
  
  <p>This does not work:</p>
  
  <pre><code>Modal.propTypes = {
  // to do how to insert nested props
  iframeProps: ({
      src: React.PropTypes.string,
      width: React.PropTypes.string,
      height: React.PropTypes.string
  }),
  modalProps: ({
      src: React.PropTypes.string,
      width: React.PropTypes.string,
      height: React.PropTypes.string,
      title: React.PropTypes.string,
      showTitle: React.PropTypes.bool,
      showOverlay: React.PropTypes.bool,
  })};
  </code></pre>