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. :-)

So the way this work, is you pass the configuration around, and doing it the way we do it your configuration has a key-value form which is `ClassName.attribute_name = value` which is automatically generated. 
If you do `ipython --help-all`  for example the list is auto generated. Here is an axample.

```
$ ipython --help-all
....
InteractiveShellApp options
---------------------------
--InteractiveShellApp.code_to_run=<Unicode>
    Default: ''
    Execute the given command string.
--InteractiveShellApp.exec_PYTHONSTARTUP=<Bool>
    Default: True
    Run the file referenced by the PYTHONSTARTUP environment variable at IPython
    startup.
--InteractiveShellApp.exec_files=<List>
    Default: []
    List of files to run at IPython startup.
--InteractiveShellApp.exec_lines=<List>
    Default: []
    lines of code to run at IPython startup.
```

Instead of `--InteractiveShellApp.exec_lines=<List>` you can also use json:
```
{
    InteractiveShellApp: {
         exec_lines=['a', 'list']
    }
}
```

(technically you can do more like validation etc, but that the basics)

Does that make sens ?