Update token request with new scope parameter

Due to Netatmo API change, a new "scope" parameter is required for client authentication.
scope is requested with "read_station" value. This is the only scope currently supported in this library. Other scope values are related to thermostat management (I don't have one, I can't test anything thus I will not try to develop in the blind.) Contributions are welcome.

Appart from new API compliance, no change has been introduced in usage.