Change to ISO 8601 date formats, without time of day.

Change to ISO 8601 date formats, without time of day.
(change-log-font-lock-keywords): Adjust accordingly.
(add-change-log-entry): Compare and insert with new date format.
(change-log-mode): Make `paragraph-start' and `paragraph-separate' use \<
like `page-delimiter' for consistency.
(change-log-time-zone-rule): New variable.
(iso8601-time-zone): New function.
(add-change-log-entry): If change-log-time-zone-rule is non-nil, calculate
date according to that rule, and indicate resulting time zone.