(Readme): Don't link to url-dav from newer Emacsen, since it doesn't
work on older versions.
(org-caldav-sync): Adapt error message.
(org-caldav-sync): Fix error message.
(org-caldav-convert-event): Enable multibyte in http process buffer (issue #8).
(url-http): Require because of removed autoloads in Emacs.