<?php
add_filter('wp_nav_menu_objects' , 'my_menu_class');
function my_menu_class($menu) {
    $level = 0;
    $stack = array('0');
    foreach($menu as $key => $item) {
        while($item->menu_item_parent != array_pop($stack)) {
            $level--;
        }
        $level++;
        $stack[] = $item->menu_item_parent;
        $stack[] = $item->ID;
        $menu[$key]->classes[] = 'level-'. ($level - 1);
    }
    return $menu;
}