Call calendar format function with moment context

Call calendar format function with moment context

What: the code:
```
moment().days(-1000).calendar(null, {
   sameElse: function() {
       return '[hoy a la' + ((this.hours() !== 1) ? 's' : '') + '] LT';
   }
});
```
Where: on http://momentjs.com/docs/#/displaying/calendar-time/ in the Chrome console.
What I got: It raises `Uncaught TypeError: this.hours is not a function`

I expected use callback like in the example on http://momentjs.com/docs/#/customization/calendar/ .

This PR makes the compatible call with https://github.com/moment/moment/blob/develop/src/lib/locale/calendar.js#L14