Refs #6 - Using class-based implementation for invitation_required decorator.
Refs #6 - Replaced some invitation_required options (forbidden, unauthorized and login) by internal (overridable) methods.
Refs #6 - Moved Ticket instance checks from TicketAuthenticationForm to invitation_required.get_ticket for smarter error handling (hard to get details about the validation errors if not form.is_valid()).
Refs #6 - Added decorators and exceptions to the API.
Refs #6 - Fixed TicketAuthenticationForm's initialization in invitation_required.get_ticket().
Refs #6 - In invitation_required decorator, restored redirection which hides credentials.
Merge pull request #9 from novapost/6-class-based-invitation-required

Closes #6 - Class-based @invitation_required