5.x support (easy!)

It seems to work fine in 5.x. I'm not sure I understand how the "help" works, but as a filter, it works. The only thing stopping people from downloading and installing is the lack of a urlclass.info file. Here's what mine looks like:

; $Id $
name = "urlclass"
description = "Adds a CSS-class to each URL according to its target."
version = "$Name$"