Added roundTo method

[triqi]

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.

[ichernev]

Can you please also add tests for all the units? I also think the code is wrong. If you round to 5 years for example, you'd assigning Math.round(2014/5) which is something like 403. Instead you should do something with the reminder (2014 % 5 --> 4) -> subtract it, then maybe add offset to round up.

[niemyjski]

sweeet!

[icambron]

Besides @ichernev's point about tests and correctness, I think we should be a little more clever with making the code terser here, e.g. that whole switch is just this.set(roundUnit(this.get(unit))).

[triqi]

@icambron yes, you have a point. I will revise this and add tests.

[triqi]

Please merge this with #1794

[icambron]

Closing in favor of #1794.