Return activities for category. If category is not specified, will
        return activities that have no category

def get_category_activities(self, category_id = None):
        """Return activities for category. If category is not specified, will
        return activities that have no category"""
        category_id = category_id or -1
        return self._to_dict(('id', 'name', 'category_id', 'category'), self.conn.GetCategoryActivities(category_id))