Verifying TTL is ambiguous regarding nanosecond handling #24

The spec's instructions for verifying token TTL doesn't explain how to handle nanoseconds.

If the user has specified a maximum age (or "time-to-live") for the token, ensure the recorded timestamp is not too far in the past.

This results in difference between implementations. For example, fernet-go uses time.Time.After, which checks equality down to the nanosecond level, while Python's cryptography.fernet truncates nanoseconds from the now timestamp before comparing. This means that a token may fail verification in one implementation, but pass in the other.