[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.
Yeah, when trying to populate the data dictionary I realized the model didn't have the notion of the schema. At first I was thinking that the list of field names could be provided when exporting to PMML but really this should be in the model itself.