Add level indicator css class to all menu items and menu links within Drupal theme

gistfile1.php

<?php

/*
 * Add level indicator css class to all menu items and menu links.
 */

function THEME_menu_link(array $variables) {
  $element = $variables['element'];
  $sub_menu = '';
  $element['#attributes']['class'][] = 'menu-item menu-item--level' . $element['#original_link']['depth'];
  $menu_link = 'menu-item-link menu-item-link--level' . $element['#original_link']['depth'];
  if ($element['#below']) {
    $sub_menu = drupal_render($element['#below']);
  }
  $output = l($element['#title'], $element['#href'], array('attributes' => array('class' => array($menu_link))));
  return '<li' . drupal_attributes($element['#attributes']) . '>' . $output . $sub_menu . "</li>\n";
}