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
maggiepint/moment,PKRoma/moment,joelmheim/moment,samuelfullerthomas/moment,ichernev/moment,samuelfullerthomas/moment,xkxx/moment,moment/moment,maggiepint/moment,joelmheim/moment,Oire/moment,PKRoma/moment,SkReD/moment,scriptrdotio/moment,xkxx/moment,moment/moment,calebcauthon/moment,Oire/moment,gabrielmaldi/moment,ze-pequeno/moment,monoblaine/moment,mj1856/moment,Oire/moment,gabrielmaldi/moment,OtkurBiz/moment,ichernev/moment,mj1856/moment,calebcauthon/moment,scriptrdotio/moment,OtkurBiz/moment,calebcauthon/moment,maggiepint/moment,scriptrdotio/moment,whytobe/moment,joelmheim/moment,moment/moment,mj1856/moment,ze-pequeno/moment,OtkurBiz/moment,julionc/moment,monoblaine/moment,julionc/moment,whytobe/moment,xkxx/moment,PKRoma/moment,samuelfullerthomas/moment,ze-pequeno/moment,julionc/moment