[SPARK-5956] [MLLIB] Pipeline components should be copyable.

This PR added `copy(extra: ParamMap): Params` to `Params`, which makes a copy of the current instance with a randomly generated uid and some extra param values. With this change, we only need to implement `fit` and `transform` without extra param values given the default implementation of `fit(dataset, extra)`:

~~~scala
def fit(dataset: DataFrame, extra: ParamMap): Model = {
  copy(extra).fit(dataset)
}
~~~

Inside `fit` and `transform`, since only the embedded values are used, I added `$` as an alias for `getOrDefault` to make the code easier to read. For example, in `LinearRegression.fit` we have:

~~~scala
val effectiveRegParam = $(regParam) / yStd
val effectiveL1RegParam = $(elasticNetParam) * effectiveRegParam
val effectiveL2RegParam = (1.0 - $(elasticNetParam)) * effectiveRegParam
~~~

Meta-algorithm like `Pipeline` implements its own `copy(extra)`. So the fitted pipeline model stored all copied stages (no matter whether it is a transformer or a model).

Other changes:
* `Params$.inheritValues` is moved to `Params!.copyValues` and returns the target instance.
* `fittingParamMap` was removed because the `parent` carries this information.
* `validate` was renamed to `validateParams` to be more precise.

TODOs:
* [x] add tests for newly added methods
* [ ] update documentation

jkbradley dbtsai

Author: Xiangrui Meng <meng@databricks.com>

Closes #5820 from mengxr/SPARK-5956 and squashes the following commits:

7bef88d [Xiangrui Meng] address comments
05229c3 [Xiangrui Meng] assert -> assertEquals
b2927b1 [Xiangrui Meng] organize imports
f14456b [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-5956
93e7924 [Xiangrui Meng] add tests for hasParam & copy
463ecae [Xiangrui Meng] merge master
2b954c3 [Xiangrui Meng] update Binarizer
465dd12 [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-5956
282a1a8 [Xiangrui Meng] fix test
819dd2d [Xiangrui Meng] merge master
b642872 [Xiangrui Meng] example code runs
5a67779 [Xiangrui Meng] examples compile
c76b4d1 [Xiangrui Meng] fix all unit tests
0f4fd64 [Xiangrui Meng] fix some tests
9286a22 [Xiangrui Meng] copyValues to trained models
53e0973 [Xiangrui Meng] move inheritValues to Params and rename it to copyValues
9ee004e [Xiangrui Meng] merge copy and copyWith; rename validate to validateParams
d882afc [Xiangrui Meng] test compile
f082a31 [Xiangrui Meng] make Params copyable and simply handling of extra params in all spark.ml components

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