[SPARK-4575] [mllib] 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

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.