pymongo-style sorting.  Accepts a list of tuples.

            :param sort_tuples: varargs of sort tuples.

def sort(self, *sort_tuples):
        ''' pymongo-style sorting.  Accepts a list of tuples.

            :param sort_tuples: varargs of sort tuples.
        '''
        query = self
        for name, direction in sort_tuples:
            field = resolve_name(self.type, name)
            if direction in (ASCENDING, 1):
                query = query.ascending(field)
            elif direction in (DESCENDING, -1):
                query = query.descending(field)
            else:
                raise BadQueryException('Bad sort direction: %s' % direction)
        return query