function initialize(buttonEl) {
  // check flag
  if (buttonEl._muiRipple === true) return;
  else buttonEl._muiRipple = true;

  // exit if element is INPUT (doesn't support absolute positioned children)
  if (buttonEl.tagName === 'INPUT') return;

  // attach event handler
  jqLite.on(buttonEl, 'touchstart', eventHandler);
  jqLite.on(buttonEl, 'mousedown', eventHandler);
}
Add ripple effects to button element.
@param {Element} buttonEl - The button element.