#24 Specify actions to take on events

Case gave me this feature request: when you make
someone a leader it sends them an email with links to
the leader's manual and whatnot.

That requires special logic to decide when someone's
group memberships change, and to these values, take
these actions.

How about a more generic event-handling mechanism? How
about every action fires an event, and the handlers are
defined in the database. If there's a handler, it is
executed. Maybe this could be used for logging, etc
etc as well.