Time zone object base class and factory
This class is the base class for all time zone objects. A time zone is
represented internally as a set of observances, each of which describes the
offset from GMT for a given time period.
Note that without the 'DateTime.pm' module, this module does not do much.
It's primary interface is through a 'DateTime' object, and most users will
not need to directly use 'DateTime::TimeZone' methods.