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. :-)

Sure, I can explain. All these `Bool`, `DottedName` and a few other are some deep metaclass magic. Any class that is a decendent of `Configurable` and have a class attribute like `ensure_ascii = Bool(True, ....` will at instanciation automatically get this class attribute transformed into a instance attribute. The `config=True` part is the thing that will more or less expose that directly in IPython config. (it's slightly complicated part so we'll probably take care of this one)