Add an event and its corresponding key to the store.

def add_event(self, key, event):
        """Add an event and its corresponding key to the store."""
        if key in self.keys or key in self.events:
            raise EventStore.EventKeyAlreadyExistError(
                "The key already existed: {0}".format(key))
        self.keys.append(key)
        # Important no exceptions happen between these lines!
        self.events[key] = event