Fixes #547 - Fixes the onLoad event for Asset.css so it should work cross

Fixes #547 - Fixes the onLoad event for Asset.css so it should work cross browser

https://mootools.lighthouseapp.com/projects/24057/tickets/547

Thanks to:
- Diego Perini's CSS Ready: http://javascript.nwbox.com/CSSReady/cssready.html
- http://www.yearofmoo.com/2011/07/mootools-asset-stylesheet-onload-event-patch.html

Tested in:
- Firefox 5
- Chrome 13
- IE6-9
- Opera 11.50