Add conversion cal->org for recurring task

[hubearth]

• Get data from rrule.
• Add recurring notation when creating org timestamp
• Add new return 'freq' to 'org-caldav-convert-event'
• Add new parameter 'freq' to 'org-caldav-insert-org-entry'
• Add new parameter 'freq' to 'org-caldav-create-time-range'

[hubearth]

In nextcloud, caldav contains some data about recurring event. They can be found in RRULE

FREQ is a string defining the 'recurring unit', like "WEEKLY" "MONTHLY" "YEARLY".
INTERVAL is an integer defining how many units separate each occurance.

In org-mode, recurring event are managed by adding a notation at the end of the time stamp (see orgmode.org).
For exemple, an event occuring each two weeks would have a caldav RRULE value of "FREQ=WEEKLY;INTERVAL=2" that is converted as "+2w" in org-mode.

As for the org->cal convertion, icalendar takes care of it all without any modification in org-caldav.el

[real-or-random]

How does this relate to #216 ?

[jackkamm]

This PR is obsolete now due to #308

However it was a useful reference for me while I was implementing #308, so thank you for submitting this originally.