Use same Attendee for each product in a product kit.

Issue Summary

I'm trying to set up uc_signup to handle a multi-day conference. Each of the 3 days is a separate product, and there is an "all days" product kit. If the user signs up for "all days" they are asked three times what their email address is (once for each product). Since the the point of the product kit is to offer a discount to a single user for purchasing a 3-day pass instead of each day product individually, it would make sense to only allow one user per product kit purchased.