Add support for GCP_OAUTH2_REFRESH_TOKEN env variable.

This addresses #8.

Also:
* Be more robust about detecting GCE service account scopes. (#5)
* Clarify different auth methods in README.md
I'm a bit confused here. I don't see the difference whether i run docker ps on container with env-file or -e passed (it's not shown in any case). At the same time both are shown with docker inspect. Maybe I'm doing something wrong though.