[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.
Right, I think this was the wrong way to fix the bug.

The bug was:
The model was training with hashingTF.numFeatures features (10, 100 or 1000), but when PipelineModel.transform() was called, HashingTF used the default numFeatures.

I probably should fix it by changing Pipeline.fit() to merge all relevant parameters from the paramMap passed to fit() into the transformers.  (The params are stored in Models but not other Transformers right now.)  I'll make that change.