small patch to atom_builder

hey - following guidelines on 'contributing to rails' page... just
letting you know I've created a small patch to give you the option of
setting the id for feed and entry elements. here's the ticket.
http://rails.lighthouseapp.com/projects/8994/tickets/485-allow-overriding-default-id-s-for-atom-feed-and-atom-entry-atom_feed_helper#ticket-485-1

Cheers,
Darragh