Support hourly repeat cookies.

Support hourly repeat cookies.

* org.el (org-repeat-re)
(org-clone-subtree-with-time-shift, org-auto-repeat-maybe)
(org-deadline, org-schedule, org-matcher-time)
(org-time-stamp, org-read-date, org-read-date-get-relative)
(org-display-custom-time, org-get-wdays)
(org-time-string-to-absolute, org-closest-date)
(org-timestamp-change): Allow to set hourly repeat cookie.
Send an error when an hourly repeat cookie is set and no hour
is specified in the timestamp.

* org.texi (Repeated tasks): Document repeat cookies for
years, months, weeks, days and hours.

* org-agenda.el (org-agenda-get-timestamps): Match hourly
repeat cookies.

* org-icalendar.el (org-print-icalendar-entries): Handle
hourly repeat cookies.

Thanks a lot to Takafumi Arakaki for this idea and for a preliminary
version of this patch.