Using script filename instead of __FILE__

The server variable script_filename contains the requested file path without symlink resolution.
__FILE__ is the 'real' file path with symlink resolution.
If a user (like me) uses a git repo for plugin management and symlinks the fever directory to the
plugin directory the inclusion and chdir'ing previously failed.
Merge pull request #6 from bountin/root_dir

Working symlinks: Using script filename instead of __FILE__