Add support for Leap Seconds.

[davesag]

From Simon on the RoRo list.

It's really just a minor detail, as NTP kind of hides leap seconds most of the time (because really, they're a pretty horrible hack), but technically speaking, sometimes the second field can contain "60" -- i.e. the minute contains 61 seconds. There's one this year, in June.

So, depending on how your system implements leap seconds, you might see:
2015-06-30T23:59:60Z