Fix for add/subtract when crossing a tz offset

When using 'add' or 'subtract' (or maybe other) moment.js functions, result was wrong if date passes through tz offset change.  See discussion in issue #28 for more details.

This commit includes a small suite of tests, along with a small fix in the spirit of that suggested by @icambron [here][1]: "If we're changing a date or month, just [...] change the offset without touching the time."

[1]: https://github.com/moment/moment-timezone/issues/28#issuecomment-25409441

This requires an associated change to the moment project (moment/moment#1419), to support this second argument (though it will not fail any harder than it currently does, if running with an older version of moment).