List of attendees for events

[sirex]

Depends on #5

It would be nice to have possibility for users to tell if they planning to attend or not. In case if no one is planning to attend, event could be cancelled automatically 4 hours before the event.

Also with this we will be able to track who is the most active in workshops.

List of attendees should be shown only for registered users. To attend to an event user should follow single link received via email.