Introduce sign_in_as helper to clean up capybara tests a bit
Changed event organizers to be a nested route and refactored controller
Moved class method 'organizer?' on EventOrganizer to instance method on Event

Refactored class method 'not_assigned_as_organizer' on User to better use railsy associations.

Tidied up names of before filters in event controller.
Refactored "volunteer_display" logic

Individual users now know whether they are only taing, or only teaching, etc.