JS/CSS file enqueues now works with secure admin

Hey Bill, thanks for the excellent plugin. In the current version of CMS Page Order, the script/style enqueues fails when the WordPress admin is running under SSL. This is because the WP_PLUGIN_URL constant doesn't support HTTPS (see http://codex.wordpress.org/Determining_Plugin_and_Content_Directories).

I've updated the CMSPO_URL to use plugin_dir_url() instead of WP_PLUGIN_URL.

Thanks again for this awesome plugin!