Kryo registrator options for Spark

This came up during a documentation review:

https://github.com/JanusGraph/janusgraph/blob/0.2/janusgraph-hadoop-parent/janusgraph-hadoop-core/src/test/resources/cassandra-read.properties#L30 Should we use:

spark.kryo.registrator=org.janusgraph.hadoop.serialize.JanusGraphKryoRegistrator

Or:

spark.kryo.registrator=org.apache.tinkerpop.gremlin.spark.structure.io.gryo.GryoRegistrator

JanusGraphKryoRegistrator only registers a serializer for Geoshape, while the TinkerPop GryoRegistrator handles many different classes. JanusGraphKryoRegistrator does not extend GryoRegistrator.

We need some clarity on how to configure the Kryo registrator to cover all required TinkerPop and JanusGraph classes. Also will need updates on the documentation and the properties files in the distribution.