Add revision comments into generated revision history

Comments are only given very basic sanitisation (to avoid breaking
Dokuwiki's file format). Mediawiki syntax used in comments is not
translated, brought over verbatim (I don't think Dokuwiki supports
markup in comments, regardless).
Fix wrong timezone offset in generated revision history.

It still seems to be confused by daylight savings time (perhaps this
is a mediawiki server misconfiguration?), however now it's off by at
most one hour as opposed to being off by UTC_OFFSET.
Add a shameless "exported by yamdwe" note to the front page of exported wikis.

I had some conflicted feelings about this, but I'm really curious to
see how much use this tool is getting. The note is added as a page
revision, so if you don't want it then it's easy to roll it back out
of your wiki after the export finishes.