Date handling.

[David Woodhouse <dwmw2@infradead.org>]

The date handling is somewhat unreliable. We render dates into textual
representation using the committer's locale (day names, etc), then later
attempt to interpret that in some other locale. And we were just using
localtime without even specifying the timezone so the timestamp was
fairly randomised anyway. In fact, an $AUTHOR_DATE environment variable
was making its way into the database entirely unchecked. 

I see two possible solutions:
	1. Just store seconds-since-GMT-epoch and if we really want, the
	   timezone as auxiliary information.
	2. Store dates in RFC2822 form.

Unless someone convincingly expresses a preference before I get to work
and start playing with it, I'll implement the latter.

-- 
dwmw2