figure out/implement key rotation

Bleeding over from #6, but unrelated to the format.

Fernet.verifier([key1, key2], token) : 👍

The verifier should expose some method for the caller to determine which key was used to generate the token, and make it easy for the developer to regenerate the token using the most recent key. Maybe also accept a developer-provided callback Fernet would call when there's a stale key.