Close Issue #18

Improve discovery of calendars in CalDAV using similar method described in 
http://code.google.com/p/sabredav/wiki/BuildingACalDAVClient#Discovery

1. check entered url is an calendar or calendar-collection
2. PROPFIND user-principal-url on url
3. PROPFIND calendar-home-set on user-principal-url
4. PROPFIND calendars on calendar-home-set

Tested with sabredav, owncloud, Yahoo.
Yahoo doesn't list calendars on PROPFIND in calendar-home-set. Therefore you have to entered the complete Calendar-URL for Yahoo. e.g.: 
https://caldav.calendar.yahoo.com/dav/<user-email>/Calendar/<calendarname>/