handle contacts URI different than ID

includes fix https://github.com/mycozycloud/cozy-webdav/issues/4
Thanks @Beanow

CardDAV tested on Android with CardDav Sync Free - Beta :
- sync contacts from server -> client
- edit contact on client -> server
- edit contact on server -> client
- create contact on client
- delete contact on client

Need tests on conflict situations
Need tests on iOS