don't import s3nb during setup.py

In case install requirements are missing, preventing them from
being discovered.
Merge pull request #3 from raphaellig/pip-fix

don't import s3nb during setup.py
Update README.md

missing quotes around the notebook url
Merge pull request #4 from patrick-russell/master

Update README.md
Add checkpoint creation
Add revert to checkpoint
Add list checkpoint so that notebooks can be reverted to checkpoints created in previous sessions
Merge pull request #5 from AustinRochford/checkpoints

Implement basic checkpoint functionality