Loader: don't rely on current working directory

If your current working directory is not the same as index.php directory, then simple relative paths will not work.
It's better and more correct to use dirname($_SERVER[PHP_SELF]) instead.

This is especially true if you can CLI type of script which is called from console like this:
```
/usr/bin/php -f /srv/www/htdocs/test/index.php >> /srv/www/htdocs/test/log/`date +"%Y%m%d"`"-index.log"
```
Where current directory is not obligatory the same where your index.php file resides.