Add UTC time class and associated functions.
Add unit test for is_leap_year() function.
Add unit test for get_utc_timestamp() function.
Add new unit tests.
Fix witching hour bug in UTC timestamp calc.

Fixed get_fuzzy_utc_timestamp() function which was failing (as
discovered by the unit test) when the UTC time being sought
corresponding to an hour that didn't exist in local time as
a result of DST beginning.