nbformat v4

Still quite a bit to do, but the implemented parts of v4 are working pretty well. I did find and fix some things while testing the validation. Both v3 and v4 have a Draft 4 jsonschema, and use the standard jsonschema format for references, so jsonpointer is no longer used.

TODO:

- [x] add convert 3<->4
- [x] make v4 current
- [x] update stream messages to `name, text`
- [x] move display_data.png to display_data.data['image/png']
- [ ] backport v4->v3 to 2.x
- [x] what's new
- [ ] what to do when uploading old notebooks (upgrade on upload, or leave alone, and upgrade on open)?
- [x] decide on UI for downgrade
- [ ] upgrade existing example notebooks
debug ?