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, this allows you to keep each configuration option at the same place as the code that needs it? But if this is the case, what do you do when the same configuration option is used in several places?

My difficulties to understand certainly originate from my own development habits: I keep all the options in a separate JSON configuration file. At launch, I read it and dispatch all the values to the different classes (I have several strategy (in DP sens) setter methods for each one). All these calls need to be specified manually, though. I guess this is a very naive approach.