Stub implementation for UDTs (Cassandra 2.1).

A proper implementation is going to be added later. Currently UDTs will come in and out as java driver core UDTValues which are not Serializable, which means you must map them into something Serializable before sending between Spark nodes or from Spark to the client. Mapping UDTValues to and from case class objects is unsupported as well.

Fixes #374.