Restricted scripts and styles to load only on plugin settings page if it is enabled.

If the Settings page has been activated, the scripts and styles will only be loaded on the settings page. If not, they will be loaded for all admin pages (like before this commit)

Also added the __FILE__ magic constant as a second parameter to the admin_enqueue_style and admin_enqueue_script, the user no longer needs to change 'plugin-name'. This also makes the code more robust.