Add entity to world.
            entity is of type Entity

def add_entity(self, entity, second=False):
        ''' Add entity to world.
            entity is of type Entity
        '''
        if not entity in self._entities:
            if second:
                self._entities.append(entity)
            else:
                entity.set_world(self)
        else:
            raise DuplicateEntityError(entity)