JulianDate.createFromIso8601 has issues

[mramato]

The current implementation of JulianDate.createFromIso8601 uses Date.parse which according to ECMAScript 5 supports ISO8601 dates. This has two major problems:

1. ISO8601 supports fractional milliseconds but Date is only accurate to the millisecond, so any conversions with fractional milliseconds are rounded and less accurate.
2. ISO8601 and JulianDate both support leap seconds, but Date does not; called Date.parse with a leap second generates an invalid date.

The solution is to simply re-implement createFromIso8601 ourselves, making sure we handle all valid date specifications. This eliminates the middle-man Date approach and maintains accuracy and leap seconds. I've added a test to Core/JulianDateSpec.js, "Construct a date from ISO8601 on a leap second.", which is currently ignored with xit that shows the problem.

[mramato]

Now that #12 has been pulled this issue is finished.