Fix strict standards issues

Hi, there are three issues:

Strict standards: Declaration of PO_Walker::start_lvl() should be compatible with Walker_Page::start_lvl(&$output, $depth = 0, $args = Array) in /.../wp-content/plugins/cms-page-order/cms-page-order.php on line 0
Strict standards: Declaration of PO_Walker::end_lvl() should be compatible with Walker_Page::end_lvl(&$output, $depth = 0, $args = Array) in /.../wp-content/plugins/cms-page-order/cms-page-order.php on line 0
Strict standards: Declaration of PO_Walker::start_el() should be compatible with Walker_Page::start_el(&$output, $page, $depth = 0, $args = Array, $current_page = 0) in /.../wp-content/plugins/cms-page-order/cms-page-order.php on line 0

This simply means, that PHP wants that the method definitions match (up to the default values – because your method is more strict: it doesn’t accept calls which are perfectly fine with the parent class because of the missing default values).

It works without it, but no reason to not include the patch, especially since it clutters the error log.

Patch is here:
https://gist.github.com/apfelbox/8538568

https://www.ads-software.com/plugins/cms-page-order/