TimeZoneTime should implement all native Date object methods

[jacobg]

Documented here or here:

• https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Date
• http://www.w3schools.com/jsref/jsref_obj_date.asp

And here is one example implementation in javascript:

      TimeZoneTime.prototype.getTimezoneOffset = function () {
        return (this.utc - this.wallTime) / 60000;
      };

[jgable]

We already keep a reference to the offset and save currently applied to a TimeZoneTime. I can use that to implement getTimezoneOffset.

I'm hesitant to implement any of the other missing toString variants like toLocaleString because they would be hacky. We must only use UTC values on the wrapped date object to ensure no daylight savings magic and most of those existing functions operate with non UTC values; see the hacky way we implement toDateString.

[jgable]

It turns out that the native getTimezoneOffset includes the current daylight savings offset in the returned value. So todays offset is different than one in February (for Chicago, US anyway).

Checkout out the new unit test for this commit for more detail.

PS. Let's make the web better by not linking to w3schools.

[jacobg]

Awesome! Thank you!