Add the ability to set the formatting language as part of the format method.

This pull-request adds the ability to configure the language as part of the format method, instead of changing it globally.

```
moment().format('LLL', { lang: 'pt' });
```

The change is primarily for node.js server side uses. When rendering a date for a client, it's not desirable to modify the global language, but only for that call.

On the server-side, this change could be used alongside the [locale](https://github.com/jed/locale) module to negotiate the correct language for displaying dates.

```
var supported = new locale.Locales(["en", "en_US", "ja", ... ]);
app.get('/', function(req, res, next) {
  var locales = new locale.Locales(req.headers["accept-language"]);
  res.render('template', { 
    localDate: moment().format('LLL', { lang: locales.best(supported) })
  });
});