function overlayOn(options, childElement) {
  var bodyEl = document.body,
      overlayEl = document.getElementById(overlayId);
    
  // add overlay
  jqLite.addClass(bodyEl, bodyClass);

  if (!overlayEl) {
    // create overlayEl
    overlayEl = document.createElement('div');
    overlayEl.setAttribute('id', overlayId);
    
    // add child element
    if (childElement) overlayEl.appendChild(childElement);

    bodyEl.appendChild(overlayEl);
    
  } else {
    // remove existing children
    while (overlayEl.firstChild) overlayEl.removeChild(overlayEl.firstChild);
    
    // add child element
    if (childElement) overlayEl.appendChild(childElement);
  }

  // iOS bugfix
  if (iosRegex.test(navigator.userAgent)) {
    jqLite.css(overlayEl, 'cursor', 'pointer');
  }

  // handle options
  if (options.keyboard) addKeyupHandler();
  else removeKeyupHandler();

  if (options.static) removeClickHandler(overlayEl);
  else addClickHandler(overlayEl);

  // attach options
  overlayEl.muiOptions = options;

  return overlayEl;
}
Turn on overlay.
@param {object} options - Overlay options.
@param {Element} childElement - The child element.