AWL API changed

Issue copied from GitHub DAViCal issue 32 submitted by abbradar:

I have an error while trying to access davical via CalDAV:

NOTICE: PHP message: PHP Fatal error:  Undefined class constant 'HttpDateFormat' in /srv/http/davical/inc/caldav-PROPFIND.php on line 113

Looks like AWL's API has changed; I don't really know PHP, so I don't know how to fix this. There is really no such constant in AwlDatabase; there is one in iCalendar, but it is marked as deprecated, so it will probably be removed soon, too.