Add a notebook option to keep non-ASCII characters

Following [this discussion on Stack Overflow](http://stackoverflow.com/questions/27186073/change-ipython-notebook-json-file-encoding) I tried to inject the ensure_ascii option of json.dump into the notebook options. Not sure if this makes the trick. This is my first pull request on GitHub, please be kind if I've done anything wrong. :-)

Yes thanks, I think I understand the advantages for the auto-detection. My only doubt was about the location, at least in the case where the same option is used in several different places. What is the criterion to decide a `Bool(...`, etc. goes here or there?

Feel free anytime to stop answering my stupid questions!