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. :-)

Technically the same option is never used in different place. If it is the case, you make an `alias` that map to many configuration option. so you can set that setting `x.Foo`  will set `z.Foo` to the same value, et `p.xpz` to `x.Foo+1`.

And there are no stupid questions !