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.. The assumption as of right now is that `Sabre\CardDAV\Card` always emits plain-old vcard blobs. This is sort of the assumption everywhere, including the backend classes.

The only 'layer' where there may be other formats, is the Plugin. The plugin is responsible to convert from and to jCard (and in the future xCard).

It will do this with incoming cards and here with outgoing cards. I can see myself wanting to also support jCard in the node/backend side in the future, but it's not the case yet.

So for now, it's just a conversion layer. Nothing changes for the implementer.