Add support for java.util.Date pickling/unpickling.

Add a Pickler/Unpickler for java.util.Date objects by converting the date information to its ISO8601 representation and converting it to UTC (not entirely sure whether this is desired?).

It might be able to write the tests in a better way, I couldn't figure out how though, but I can't say I really like them, especially the date generator function, feedback most welcome.