Feature State Machine for Rsvp

Implemented different types of Rsvp using subclasses with
different state machines.

When creating an event you have to decide which type of
Rsvp you want to use. Currently two are implmented:

OpenRsvp = anyone can rspv
RsvpWithWaitlist = anyone can rsvp, until the maximum number of attendees is reached, then a waitlist is kept in order of rsvp.  handling the waitlist (moving people up) is not implemented yet.

i18n mostly done, but not perfect.