[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.
This is a notable issue to discuss -- the MLlib models don't have any indication of what the fields are. Although you can make up a "field_X" name for them it kind of makes the resulting model unusable. I think this may require later changes that may be coming in MLlib where MLlib might have more of a notion of schema.