Improve GraphSON representation of java.time.* classes

Currently the java.time classes use the default field serializer which generates stuff like this (e.g. for Instant):

{"nano":657000000,"epochSecond":1454552791}

it would be nicer if we just did ISO-8061 in this case:

2016-02-04T02:26:31.657Z

Do nicer/smarter things for other classes as well.