Adding settings to configure relative time thresholds between time units

This new feature allows a user to set custom thresholds for the division between time units for relative time strings.

Example:
The default threshold between hours and days as the unit for relative time is 22 hours. With this feature the user can now set a custom threshold like so:
```
var m = moment().subtract('hours', 23);
m.fromNow(); // Shows "a day ago"
moment.relativeTimeThreshold('h', 24)
m.fromNow(); // Shows "23 hours ago"
```