Clean up date and timezone handling.
We used to pass changelog dates around as a "unixtime timezone" string
containing a pair of encoded ints. Now, they get passed around as a
(unixtime, timezone) tuple of numbers, which makes much more sense.
<item>
<title>#desc|firstline|escape#</title>
<link>#url#?cmd=changeset;node=#node#</link>
<description><![CDATA[#desc|escape|addbreaks#]]></description>
<author>#author|obfuscate#</author>
<pubDate>#date|rfc822date#</pubDate>
</item>