Added emailing and fixed RSVPing.

hVite will now send emails. Dumping the data for these emails suggests that they are made correctly. However, this has not been put through a live test (yet). Users can now RSVP to events. They are given links to do so in emails they receive as invitations. Entries in hvite_rsvp are added for all users when events are created. No functionality exists for adding more users to an event yet; however, events can be modified.