International dates in Jupyter savewidget

The code in notebook/static/edit/js/savewidget.js uses the `moment.js` javascript library which, unfortunately, uses locales to display dates. The problem is that the default locale is the US one, which is very confusing if you are not used to it (months/days are switched, days starts at 12, 12 pm does not follow 11 pm, etc...).

Here is the javascript code to make Jupyter display dates and time following the [ISO8601 international date/time format](https://en.wikipedia.org/wiki/ISO_8601).

``` javascript
moment.locale('en', {
longDateFormat : {
LT : 'HH:mm',
LTS : 'HH:mm:ss',
L : 'YYYY-MM-DD',
LL : 'D MMMM YYYY',
LLL : 'D MMMM YYYY HH:mm',
LLLL : 'dddd, D MMMM YYYY HH:mm'
},
week : {
dow : 1, // Monday is the first day of the week.
doy : 4 // The week that contains Jan 4th is the first week of the year.
}
});
```