CardDAV contacts shoudl have etag as one of the attributes. Impelment get/set methods
Fetch and track contact etags during prep_sync_lists
Support reading/writing last modification time from vCard entires on CardDAV server
Implement a del_folder() method for pimdb_cd to clear our entries from a given folder
Implement batch_create() method for CardDAV
Modify get_updated() to return a ISO formatted timestamp if possible
format conversion routines from/to datetime.datime and vCard REV timestamp format
Ensure the sync tags get tracked and written to the right places on sync. batch_update() implemented