function initialize(inputEl) {
  // check flag
  if (inputEl._muiTextfield === true) return;
  else inputEl._muiTextfield = true;

  if (inputEl.value.length) jqLite.addClass(inputEl, notEmptyClass);
  else jqLite.addClass(inputEl, emptyClass);

  jqLite.on(inputEl, 'input', inputHandler);

  // add dirty class on focus
  jqLite.on(inputEl, 'focus', function(){jqLite.addClass(this, dirtyClass);});
}
Initialize input element.
@param {Element} inputEl - The input element.