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. :-)

ok, so you probably want to move this [here](https://github.com/ipython/ipython/blob/master/IPython/nbformat/v4/nbjson.py#L44-L57), 

For it to work you want `NotebookWriter` (in same folder, `rwbase.py)` to inherit from `IPython.config.Configurable` (some metaclass magic) and pass `ensure_ascii=self.ensure_ascii` in `json.dumps`.

Probably not enough but I need to dig a bit to find the rest that needs to be modified.