Retrieve details on a single extension.

def retrieve_extension(self, name, **kw):
        """Retrieve details on a single extension."""

        response = self.request(E.retrieveExtensionRequest(
            E.name(name),
            E.withDescription(int(kw.get('with_description', 0))),
            E.withPrice(int(kw.get('with_price', 0))),
            E.withUsageCount(int(kw.get('with_usage_count', 0))),
        ))

        return response.as_model(Extension)