Make module more themable

This patch renames the megamenu_theme_menu_tree function to theme_megamenu_menu_tree and adds a reference to it in the megamenu_theme function. This allows the megamenu _menu_tree function to be overwritten by a theme's template.php file.

Setting up support for tpl files would be better, but this does the job, allowing themers to modify output without changing the module files.