Initialize application object.

def init_app(self, app, entry_point_group='invenio_celery.tasks',
                 **kwargs):
        """Initialize application object."""
        self.init_config(app)
        self.celery = FlaskCeleryExt(app).celery
        self.entry_point_group = entry_point_group
        app.extensions['invenio-celery'] = self