Add RSVP object to mail params

I see that the mail parameters for drupal_mail() are generated via rsvp_mailer_replacements(). The way this function is used seems a bit strange to me, but that's for another issue.

I think it would be very useful to pass along the RSVP object. Otherwise, modules implementing hook_mail_alter() have no data to really work with.

Patch attached does just that.