allow more flexible theming of nice menus

attached patch abstracts the theming functions for nice menus a bit more. this opens up a lot more possibilities, like using the nice menu logic for primary/secondary links in a theme.

here's what i've done:

• added an optional $menu argument to the relevant theming functions. this allows a power themer to pass a custom menu tree to the theming function. probably won't be used all that often, but nice to have :)
• created a general theming function for wrapping the final ul's. this also controls the nice menu id and the direction.
• cleaned up the block code to use the new theming functions.
• added a theme_primary_nice_menu_links() function. this is basically a helper function for themers to add primary nice menu links.

tested with both regular block nice menus, and with primary links nice menus at http://dominatefantasyfootball.net (hover over 'Blogs'). works beautifully in both cases as far as i can tell.