[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 wasn't sure what to put here.
Rattle put DMG
http://www.dmg.org/pmml_examples/rattle_pmml_examples/IrisKMeans.xml
while KNIME put KNIME
http://www.dmg.org/pmml_examples/KNIME_PMML_4.1_Examples/single_iris_kmeans.xml

Should I change it to "Apache Spark"?