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. :-)

`self.ensure_ascii` will not be defined until you moved your piece of code from `IPython/html/notebookapp.py` to this file line 45. (You might also need the Bool import)