Confirm email addresses

[jamezpolley]

Two reasons:

• So that we can be confident that people own the email address their openid says they do; in case we ever want to mail them. I can't think why we would want to do this, it just sounds nifty.
• To handle the case where a person is invited by someone; and later registers their attendance themselves - we should be able to show them their previous invitation.
• This can probably be skipped for certain providers - eg, google, yahoo