Adding moment.to (similar to moment.from)

Newly added `moment.to` is similar to `moment.from`, but the result is reversed as the relativity of dates are considered.

```
var today = moment();
var dayafter = moment(today).add(2, 'd');

console.log(today.from(dayafter));       // 2 days ago
console.log(today.to(dayafter));         // in 2 days
console.log(today.to(dayafter, true));   // 2 days
```