Use time.Time in the Signature struct

The antisymmetry of `sig.When.Unix()` vs `sig.Offset()` isn't great, but getting the offset in seconds becomes annoying otherwise, as `time.Time` wants to give you the timezone's name as well, and gives you the offset in seconds, while we want it in minutes.

Do we want a `sig.Unix()` function that just returns `sig.When.Unix()`?