storage/dav: utilize calendar-order property

Proposed at pimutils#836

Author: dilyanpalauzov

vdirsyncer/storage/dav.py

@@ -729,6 +729,7 @@ class CalDAVStorage(DAVStorage):
     _property_table.update({
         'color': ('calendar-color', 'http://apple.com/ns/ical/'),
         'description': ('calendar-description', 'urn:ietf:params:xml:ns:caldav'),
+        'calendar-order': ('calendar-order', 'http://apple.com/ns/ical/'),
     })
 
     def __init__(self, start_date=None, end_date=None,