[PATCH] go: add binding for notmuch_message_get_date

[Trevor Jim]

> Given the current troubles with the main library API and missing status
> values, I'd say in the long run we'll be happier with a real status
> return. OTOH, I don't much about go.

I'm not sure what the "current troubles" are (happy to hear more), but
the way I've handled the nil case is in keeping with the way it's
handled in the other function bindings for go.  Adding an error return
to all relevant functions seems like a big change that should be handled
separately, yes?

In defense of the current strategy, any caller can check for the nil
case before calling.

-T