#2 time-based publishing

[Anonymous]

Implement time-based publishing. Implementation outline:
* add as an optional feature for WEprojectinfo.pm
* version view (e.g. as a select-box with labels
"version v1.1 - 2003-12-12" or "version v1.2 -
released/active")
* add a field releasedate (yyyy-mm-dd), check for
correct value, maybe add also a field releasetime (hh:mm)
* add a new field PublishAt to WE::Obj
* fire an "at" command to update the site
* admin interface: add links "preview site at date"
and "preview site at next change"
* release/publish functions: optional date parameter to
simulate a site at the given date