No calDAV and iCalendar support

A standalone calendar is rather useless, so it needs to support the most prevalent and open standards for exchanging calendar data:

- CalDAV for syncing to the cloud (or your own server):
    https://en.wikipedia.org/wiki/CalDAV
    http://caldav.calconnect.org/
- iCAL for fetching calendars from the web (or importing calendar data)
    https://en.wikipedia.org/wiki/ICalendar
    https://tools.ietf.org/html/rfc5545