minor changes
Minor fixed to TimeZone
added ZoneDateTime.dateTime method to get DateTime from ZoneDateTime.

The method is intended to get local DateTime value of the ZoneDateTime
in the specified time zone.
Made Instant enumerable over it's millisecondsOfEra value
+ OffsetTimeZone.offsetMillisecons is now shared 
+ timeZone.offset(Integer, Integer, Integer) now returns OffsetTimeZone
Updated tests for ZoneDateTime, Periods and Instant