Fix for entry_with_uuid_exists? method, added "url" property

Hi,

i could not get the lib to work with radicale server, it suddenly stopped creating events throwing DuplicateErrors. After some digging I narrowed the source of the error down to the Icalendar.parse-call in the entry_with_uuid_exists? method in lib/agcaldav/client.rb, which returns an empty array if the preceding request returns an HTTP Error 410.

I've also added the 'url' property to create_event.

Thanks in advance!