[PATCH] go: add binding for notmuch_message_get_date

[Trevor Jim]

> -1 is 1969-dec-31 23:59:59 UTC
>
> Should we care -- and e.g. return status in separate return value ?

BTW, I notice that when a message has no Date: field, notmuch reports its
timestamp as 0 without reporting an error :-)

-Trevor