function setupPostMessage() {

		if( config.postMessage ) {
			window.addEventListener( 'message', onPostMessage, false );
		}

	}
Registers a listener to postMessage events, this makes it
possible to call all reveal.js API methods from another
window. For example:

revealWindow.postMessage( JSON.stringify({
  method: 'slide',
  args: [ 2 ]
}), '*' );