Implement leap seconds as an option

[benfitzpatrick]

UTC allows for insertion of extra leap seconds (or removal of existing seconds) based on the variation of the Earth's rotational period with respect to the ICRF.

This means that some days do not have 86400 seconds, some hours do not have 3600 seconds, and some minutes do not have 60 seconds. For example, the sequence of seconds in UTC following 2012-06-30T23:59:58 begins with:

2012-06-30T23:59:59
2012-06-30T23:59:60
2012-07-01T00:00:00

Positive or negative leap second application is not particularly predictable and is governed by physical observations of the Earth's rotational period - however, there are historical leap seconds that we could add if we wished.

This would definitely have to be an option which defaults to off, as leap seconds can cause severe problems for other software. Leap second smearing is beyond the scope of this library, but is another approach.