create and index event changes.
send change email.
update settings example.
fixed: rails ignore bcc email attributes.
don't use bcc, the bcc users will see the first user(to)'s email address.'
fixed rspec error.
Merge pull request #378 from saberma/saberma-feature/358-change-notification

主办者可以向所有报名用户发送临时变更通知