[SPARK-4575] [mllib] [docs] spark.ml pipelines doc + bug fixes

Documentation:
* Added ml-guide.md, linked from mllib-guide.md
* Updated mllib-guide.md with small section pointing to ml-guide.md

Examples:
* CrossValidatorExample
* SimpleParamsExample
* (I copied these + the SimpleTextClassificationPipeline example into the ml-guide.md)

Bug fixes:
* PipelineModel: did not use ParamMaps correctly
* UnaryTransformer: issues with TypeTag serialization (Thanks to @mengxr for that fix!)

CC: @mengxr @shivaram  @etrain  Documentation for Pipelines: I know the docs are not complete, but the goal is to have enough to let interested people get started using spark.ml and to add more docs once the package is more established/complete.
Thanks! I see the issue now. Could we create a separate PR? This is not a blocker for the release, and it might need some discussion.