Merge pull request #3158 from minrk/autosave Add autosaving to the notebook, while retaining manual checkpoint creation. Autosave will perodically save the notebook, with a default interval of two minutes. This interval is configurable, and the system will automatically back off the frequency if it detects that the save operation takes too long. This can be the case if working remotely over a slow link, so this will automatically conserve bandwidth. The manual save operation remains available and bound to the same keys, but now it causes the creation of a separate checkpoint, which is stored in a hidden directory called `.ipynb_checkpoints`. This provides users a poor-man's version control with a single revert point and the ability to safely experiment with potentially destructive changes to a notebook without concern that the autosave operation may clobber their file on disk. See [IPEP 15](https://github.com/ipython/ipython/wiki/IPEP-15%3A-Autosaving-the-IPython-Notebook) for details of the proposal.