nbformat v4 draft schema

This is a work-in-progress pull request for a draft of the nbformat v4 JSON schema. Following [IPEP 17](https://github.com/ipython/ipython/wiki/IPEP-17%3A-Notebook-Format-4), this pull request addresses the first task:

1. Define v4 in a doc (not just changes, full spec - v3 was never fully defined)

and should implement the following changes:

- [x] remove multiple worksheets
- [ ] use mime-type output keys
- [ ] remove python-centric names
- [x] make cell content key uniform
- [ ] metadata changes