[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.
`$k = 10$` is common when the number of instances is small. For big data, maybe it doesn't worth the trade-off.