Added "url" property to create_event
Added call to errorhandling to entry_with_uuid_exists? method

GET-Request to nonexistent UUID results in HTTP Error 410.
Icalendar.parse returns an emtpy array and thus a
AgCalDAV::DuplicateError ist thrown in create_event although the uuid
does not exist
Changed delete_event to return true for all successful delete operations

delete_event returned false for succesful operations (Codes in the 2xx
range)
Merge pull request #3 from leobossmann/master

Fix for entry_with_uuid_exists? method, added "url" property