Start all services of a given class. If this manager doesn't already
        have a service of that class, it constructs one and starts it.

def start_class(self, class_):
        """
        Start all services of a given class. If this manager doesn't already
        have a service of that class, it constructs one and starts it.
        """
        matches = filter(lambda svc: isinstance(svc, class_), self)
        if not matches:
            svc = class_()
            self.register(svc)
            matches = [svc]
        map(self.start, matches)
        return matches