Handle multiple email addresses for a user

[pat]

Many folks sign up for conferences and camps using a mixture of work and personal email addresses, and so they get invitations for joining Ruby Australia as a member more than once.

It'd be nice if we could allow people to associate multiple email addresses with their membership. We'd want to confirm each email address, and then modify the invitation system so it's smart enough to skip over people if any of their email addresses are in the system.

It'd also be good to allow people, upon getting an extra invite, to have a one-click solution for associating that email address with their existing account (I'm guessing: link in email, which prompts a sign-in, which then allows people to click a button to confirm the association).