Vcard convert

Work in progress.

- [x] Listening to Accept headers
- [x] Converting to vCard 3.0, 4.0, jCard on `GET`
- [ ] Updating supported-address-data
- [ ] Converting from jCard from `PUT`
- [ ] Supporting new formats in `REPORT`
- [ ] All new code unittested
This is true, but that path can also never be taken in the source. I could remove the `is_null($result)` and add the `default :` clause though.