Adhere to specs

[kokes]

There is a clear explanation of the spec on the nbformat page. There's also a JSON spec for both v3 and v4. There's a few things we're not supporting - collapsed, scrolled, errors, some mime types (especially in v3).

Steps now

1. Skim the specs and find out what is not supported. List it here to check off.
2. Split out to separate issues if quite big - eg JavaScript support is probably more complicated than eval, or is it?
3. Make sure we err when not supporting something - console.error at least, possibly in the DOM as well.
4. Track the changelog to find out what's changed.