Adding ability to set the parse language

This is to support #508 so that you can set a parsing language for in input string + format.

This way, you don't have to change the global language to parse a moment into another language. Without a language key, it defaults to the current behavior using the global language.

```javascript
moment('2012 July', 'YYYY MMM', 'en');
moment('2012 juillet', 'YYYY MMM', 'fr');
```