backport nbformat v4 to 2.x
add v4 to reader
enable reading v4 notebooks

- validation is not backported
- update some tests to reflect this
- test opening v4 notebooks
fixup
Set orig_nbformat keys when downgrading v4->v3
don't write orig_nbformat to disk in v3
don't assume converted notebooks are old

in notebook.js dialogs, which can now be triggered
by nbformat 4 from the future.