Migrate custom date objects and logic to Java 8's Date / Time API

Rather than using the custom-rolled DateTime, Period, Dur, etc. objects, migrate to using Java 8’s Date / Time API (Instant, Period, Duration, etc.).

[benfortuna]

Migration to new date/time API will be an extensive project and will likely see a number of alpha releases prior to completion (propose to use 3.1.x release versioning before final release of 4.x).

Points to note as part of migration:

• Properties that support dates can typically switch between DATE/DATE-TIME values via changing the Value parameter. To support this we require a class to encapsulate a Temporal (date) value and a format flag to indicate the external representation. Propose TemporalAdapter to support parsing/formatting Temporal values
• Some properties require UTC-based DATE-TIME values. For these we use generics to enforce Instant as the Temporal type such that local/zone-based values aren’t permitted.
• For classes supporting data types/logic (e.g. Recur) we should use generics where possible to support applicable Temporal types (i.e. Instant = UTC time, OffsetDateTime = zone-based time without DST rules, etc., ZonedDateTime = DST rules, LocalDate = date-only values)

[benfortuna]

v4 now in beta.