[SPARK-8538] [SPARK-8539] [ML] Linear Regression Training and Testing Results

Adds results (e.g. objective value at each iteration, residuals) on training and user-specified test sets for LinearRegressionModel.

Notes to Reviewers:
 * Are the `*TrainingResults` and `Results` classes too specialized for `LinearRegressionModel`? Where would be an appropriate level of abstraction?
 * Please check `transient` annotations are correct; the datasets should not be copied and kept during serialization.
 * Any thoughts on `RDD`s versus `DataFrame`s? If using `DataFrame`s, suggested schemas for each intermediate step? Also, how to create a "local DataFrame" without a `sqlContext`?

Author: Feynman Liang <fliang@databricks.com>

Closes #7099 from feynmanliang/SPARK-8538 and squashes the following commits:

d219fa4 [Feynman Liang] Update docs
4a42680 [Feynman Liang] Change Summary to hold values, move transient annotations down to metrics and predictions DF
6300031 [Feynman Liang] Code review changes
0a5e762 [Feynman Liang] Fix build error
e71102d [Feynman Liang] Merge branch 'master' into SPARK-8538
3367489 [Feynman Liang] Merge branch 'master' into SPARK-8538
70f267c [Feynman Liang] Make TrainingSummary transient and remove Serializable from *Summary and RegressionMetrics
1d9ea42 [Feynman Liang] Fix failing Java test
a65dfda [Feynman Liang] Make TrainingSummary and metrics serializable, prediction dataframe transient
0a605d8 [Feynman Liang] Replace Params from LinearRegression*Summary with private constructor vals
c2fe835 [Feynman Liang] Optimize imports
02d8a70 [Feynman Liang] Add Params to LinearModel*Summary, refactor tests and add test for evaluate()
8f999f4 [Feynman Liang] Refactor from jkbradley code review
072e948 [Feynman Liang] Style
509ae36 [Feynman Liang] Use DFs and localize serialization to LinearRegressionModel
9509c79 [Feynman Liang] Fix imports
b2bbaa3 [Feynman Liang] Refactored LinearRegressionResults API to be more private
ffceaec [Feynman Liang] Merge branch 'master' into SPARK-8538
1cedb2b [Feynman Liang] Add test for decreasing objective trace
dab0aff [Feynman Liang] Add LinearRegressionTrainingResults tests, make test suite code copy+pasteable
97b0a81 [Feynman Liang] Add LinearRegressionModel.evaluate() to get results on test sets
dc51bce [Feynman Liang] Style guide fixes
521f397 [Feynman Liang] Use RDD[(Double, Double)] instead of DF
2ff5710 [Feynman Liang] Add training results and model summary to ML LinearRegression

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