Better timezone support for manipulating moments

This adds a second argument to setters that provides a hint to updateOffset to not change hours/minutes when crossing a tz boundary.  This is frequently desirable when modifying part of an existing moment object directly.

It further passes this argument whenever adding / subtracting days or months, fixing an issue with moment-timezone (moment/moment-timezone#28).  See the corresponding PR for moment-timezone here: moment/moment-timezone#53.