Set plural form of taxonomy name as its name and menu_name

WordPress uses plural form of taxonomy name as a label in WP Admin left menu and in header on taxonomy page (https://github.com/WordPress/WordPress/blob/master/wp-includes/taxonomy.php#L474):
![00000731](https://cloud.githubusercontent.com/assets/166641/4334137/e52dc0e0-3fe9-11e4-9bff-f4eaf1093d29.png)

I applied the same convention for `custom-tax` feature, as currently singular form of taxonomy name is used in those areas.