Rename transformer and transformer builder classes

[vruusmann]

Current names org.jpmml.evaluator.spark.Transformer and o.j.e.s.TransformerBuilder are too vague - there is no clear indication that they deal with the PMML data format (as implemented by the JPMML-Evaluator library). Perhaps adding a common prefix such as "PMML" (or "Evaluator") would be all that is needed.

Renaming classes is a breaking API change, so it would necessitate upgrading the JPMML-Evaluator-Spark base version from 1.2.X to 1.3.X.