Support for upward expanding menus

I created support for upward expanding menus. Added "up" in the module itself. Modified the css file:

• Removed the top declaration from the ul.nice-menu ul so that each kind of menu can define the vertical position
• created the nice-menu-up css (mostly a copy of the down css, but has a different top value)

I tested it in safari, firefox and ie6, works for me.