Thread-safe and non-blocking factories

• From now the default Period and Time factories are thread-safe.
• TimeFactory and PeriodFactory are interfaces. Not structs.
• NewNonblockingTimeFactory, NewNonblockingPeriodFactory methods are also available now.
• Some test fixes

License

v1.0.2

LICENSE

IsZero methods of Time and Period

• IsZero method of Time
• IsZero method of Period
• Documentation minor fixes
• Code coverage

The first public version

v1.0.0

godoc link & fix typo