[SPARK-1406] Mllib pmml model export 

See PDF attached to the JIRA issue 1406.

The contribution is my original work and I license the work to the project under the project's open source license.
I assume you are referring to 
JAXBUtil.marshalPMML(pmml, new StreamResult(outputStream));
which takes as param the StreamResult object and for which there are many constructors.

I didn't want to expose the StreamResult as it is xml specific, but if the decision is to only support PMML xml we may add also the StreamResult.

Personally I would still like a simple saveLocalFile(path: String) so that the user doesn't have to think too much and just call:
ModelExportFactory.createModelExport(clusters,ModelExportType.PMML).saveLocalF
ile("/tmp/kmeans.xml") for all models.