PhpThumb: configuration and security mode

Hello

will add new file:
phpThumb.config_OVERRIDE.php

so that users can change default PHPThumb configuration without worring of configuration file being overwritten on every upgrade

Thus it will be usable for
- website that DOCUMENT ROOT is not detected correctly
- also to override PHPThumb default cache size (e.g. needed for very large websites), etc
- other configuration