Validation

Updated ValidationError format and added gettext for translation and a
descriptive error code
Unique Email per Event

Added unique_together on Guest model for event and email fields.
Prevents error when saving RSVP if there are two entrees for the email
address.
URL template tag

Updated URL template tag syntax
CSRF Token

Added CSRF Token template tag to form
import urls

Changed urls import statement (.defaults removed in 1.6)