Introduce JDK8 Date and Time

I started these changes, but stashed them away when @purikas started his job on PR #374. As I see that work is a little stalled I thought I'd chip in with my work – for use or inspiration ...

It is pretty straight forward and should be (pretty) complete, but some comments:

* JPA 2.x has (as I see it) currently no native support for `LocalDateTime`  so I have introduced a `LocalDateTimePersistenceAttributeConverter`
* There are some date conversion going on in the views, hence the `ViewHelper`
* For search with Jest I have added a serializer/deserializer that should behave (wrongly) like it is today. This must be adapted in synch with issue #421 (see comment in code)
* I have replaced `java.util.Date` with `LocalDateTime` in the domain classes. There may be a use case for `LocalDate` in there somewhere, but I didn't see it. If so, another attribute converter must be added (see bullet 1).
 
I have signed the CA.