Makes rule 'name' available to all subsequently loaded Jamfiles.

        Calling that rule wil relay to 'callable'.

def add_rule(self, name, callable_):
        """Makes rule 'name' available to all subsequently loaded Jamfiles.

        Calling that rule wil relay to 'callable'."""
        assert isinstance(name, basestring)
        assert callable(callable_)
        self.project_rules_.add_rule(name, callable_)