add 'first', 'odd', 'even' to listing

Hi

This patch adds 'first', 'odd', 'even' classes to ever li tag. That helps a lot when styling the menus.
E.g. zebra stripes or borders between elements.

I know I can do that by overriding your theme function, but I think this would be quite useful in many cases so I created a patch for nice_menus.module.
Would be nice to get that into the module for good.