authorization and credential storage

[rexim]

Split from #14
TODO bc02b696-9ad8-4572-8faa-58caed05f612

---

Develop a mechanism for storing and retrieving credentials for ytservice_from_credentials() and authorizing the Tsoding Tools to get those credentials in the first place.

For more info see https://developers.google.com/api-client-library/python/guide/aaa_oauth