[SPARK-11217][ML] save/load for non-meta estimators and transformers

This PR implements the default save/load for non-meta estimators and transformers using the JSON serialization of param values. The saved metadata includes:

* class name
* uid
* timestamp
* paramMap

The save/load interface is similar to DataFrames. We use the current active context by default, which should be sufficient for most use cases.

~~~scala
instance.save("path")
instance.write.context(sqlContext).overwrite().save("path")

Instance.load("path")
~~~

The param handling is different from the design doc. We didn't save default and user-set params separately, and when we load it back, all parameters are user-set. This does cause issues. But it also cause other issues if we modify the default params.

TODOs:

* [x] Java test
* [ ] a follow-up PR to implement default save/load for all non-meta estimators and transformers

cc jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #9454 from mengxr/SPARK-11217.