Add category to bodyclass

gistfile1.php

<?php

add_filter( 'body_class', 'fst_category_id_class' );
/**
 * This function add the category name to the bodyclass.
 * 
 * @access public
 * @author Remkus de Vries
 * @param mixed $classes
 * @return void
 */
function fst_category_id_class( $classes ) {
	global $post;
	
	foreach( ( get_the_category( $post->ID ) ) as $category )
		$classes[] = $category->category_nicename;
	return $classes;
}