Date handling.

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.