Changed DateTime\Period to only store years, months and days
Added plusYears(), plusMonths() and plusDays() to DateTime\Period
Added DateTime\Period::isZero()
Implemented missing functionality and refactored DateTime\LocalDate
Added nanosecond support to LocalTime::ofSecondOfDay()

Added unit tests to LocalTime factory methods.
Added nanosecond support to LocalDateTime, removed toEpochSecond()

toEpochSecond() does not belong there as it needs a time-zone.