function popup(url, name, width, height, options)
{
  var x = (screen.width - width) / 2,
    y = (screen.height - height) / 2
    ;

  options +=  ', left=' + x +
        ', top=' + y +
        ', width=' + width +
        ', height=' + height
    ;
  options = options.replace(/^,/, '');

  var win = window.open(url, name, options);
  win.focus();
  return win;
}
Opens up a centered popup window.
@param {String} url    URL to open in the window.
@param {String} name   Popup name.
@param {int} width   Popup width.
@param {int} height    Popup height.
@param {String} options  window.open() options.
@return {Window}     Returns window instance.