Better late than never. Added simple credits, workdays(years), and license notificati...

Better late than never. Added simple credits, workdays(years), and license notification desired by GPL.
* Licnese is GPL, especially 'version 2 or (at your option) any later version'. See the header of lib/pukiwiki.php (or header of old version's pukiwiki.php).
* Years by PukiWiki Developers Team are only clear ones roughly trackable from cvs repositories.
* Originally sng created PukiWiki at 2001, so more files and/or codes may be written at 2001.