[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 think this conflates the idea of being a model with serializing a model to a particular form or place. For example this limits to files, whereas JPMML has one-liners for serializing to a bunch of things already. I think this could be punted to caller code, or at least an orthogonal helper class, not part of the export type hierarchy.