Feature - Round to a unit by a given offset

Round up, down or to the nearest unit.

Usage: moment.roundTo(units, [offset, midpoint])

m.roundTo('minute', 15); // Round the moment to the nearest 15 minutes.
m.roundTo('minute', 15, 'up'); // Round the moment up to the nearest 15 minutes.
m.roundTo('minute', 15, 'down'); // Round the moment down to the nearest 15 minutes.
Maybe I wasn't clear, but L1989 could simply be removed. `m.roundTo(u, o, someVar ? "down" : undefined)` will have the same effect (or anything else instead of `undefined`).

I've noticed this because jshint complains because of this useless `case 'nearest':`