category slug functions

@elzii In this commit you'll find the main functions for grabbing category slugs. The function `get_yearbook_entry_category_slug( $post_id );` will neatly grab and output your slug. You just need to pass an ID to it. 

`throwback_cat_body_class();` adds the slug to the `body_class()` filter hook. It only runs on the single.php template and if there is a category. There will always be a category.