[SPARK-1545] [mllib] Add Random Forests

This PR adds RandomForest to MLlib.  The implementation is basic, and future performance optimizations will be important.  (Note: RFs = Random Forests.)

# Overview

## RandomForest
* trains multiple trees at once to reduce the number of passes over the data
* allows feature subsets at each node
* uses a queue of nodes instead of fixed groups for each level

This implementation is based an implementation by @manishamde and the [Alpine Labs Sequoia Forest](https://github.com/AlpineNow/SparkML2) by @codedeft (in particular, the TreePoint, BaggedPoint, and node queue implementations).  Thank you for your inputs!

## Testing

Correctness: This has been tested for correctness with the test suites and with DecisionTreeRunner on example datasets.

Performance: This has been performance tested using [this branch of spark-perf](https://github.com/jkbradley/spark-perf/tree/rfs).  Results below.

### Regression tests for DecisionTree

Summary: For training 1 tree, there are small regressions, especially from feature subsampling.

In the table below, each row is a single (random) dataset.  The 2 different sets of result columns are for 2 different RF implementations:
* (numTrees): This is from an earlier commit, after implementing RandomForest to train multiple trees at once.  It does not include any code for feature subsampling.
* (feature subsets): This is from this current PR's code, after implementing feature subsampling.
These tests were to identify regressions in DecisionTree, so they are training 1 tree with all of the features (i.e., no feature subsampling).

These were run on an EC2 cluster with 15 workers, training 1 tree with maxDepth = 5 (= 6 levels).  Speedup values < 1 indicate slowdowns from the old DecisionTree implementation.

numInstances | numFeatures | runtime (sec) | speedup | runtime (sec) | speedup
---- | ---- | ---- | ---- | ---- | ----
 | | (numTrees) | (numTrees) | (feature subsets) | (feature subsets)
20000 | 100 | 4.051 | 1.044433473 | 4.478 | 0.9448414471
20000 | 500 | 8.472 | 1.104461756 | 9.315 | 1.004508857
20000 | 1500 | 19.354 | 1.05854087 | 20.863 | 0.9819776638
20000 | 3500 | 43.674 | 1.072033704 | 45.887 | 1.020332556
200000 | 100 | 4.196 | 1.171830315 | 4.848 | 1.014232673
200000 | 500 | 8.926 | 1.082791844 | 9.771 | 0.989151571
200000 | 1500 | 20.58 | 1.068415938 | 22.134 | 0.9934038131
200000 | 3500 | 48.043 | 1.075203464 | 52.249 | 0.9886505005
2000000 | 100 | 4.944 | 1.01355178 | 5.796 | 0.8645617667
2000000 | 500 | 11.11 | 1.016831683 | 12.482 | 0.9050632911
2000000 | 1500 | 31.144 | 1.017852556 | 35.274 | 0.8986789136
2000000 | 3500 | 79.981 | 1.085382778 | 101.105 | 0.8586123337
20000000 | 100 | 8.304 | 0.9270231214 | 9.073 | 0.8484514494
20000000 | 500 | 28.174 | 1.083268262 | 34.236 | 0.8914592826
20000000 | 1500 | 143.97 | 0.9579634646 | 159.275 | 0.8659111599

### Tests for forests

I have run other tests with numTrees=10 and with sqrt(numFeatures), and those indicate that multi-model training and feature subsets can speed up training for forests, especially when training deeper trees.

# Details on specific classes

## Changes to DecisionTree
* Main train() method is now in RandomForest.
* findBestSplits() is no longer needed.  (It split levels into groups, but we now use a queue of nodes.)
* Many small changes to support RFs.  (Note: These methods should be moved to RandomForest.scala in a later PR, but are in DecisionTree.scala to make code comparison easier.)

## RandomForest
* Main train() method is from old DecisionTree.
* selectNodesToSplit: Note that it selects nodes and feature subsets jointly to track memory usage.

## RandomForestModel
* Stores an Array[DecisionTreeModel]
* Prediction:
 * For classification, most common label.  For regression, mean.
 * We could support other methods later.

## examples/.../DecisionTreeRunner
* This now takes numTrees and featureSubsetStrategy, to support RFs.

## DTStatsAggregator
* 2 types of functionality (w/ and w/o subsampling features): These require different indexing methods.  (We could treat both as subsampling, but this is less efficient
  DTStatsAggregator is now abstract, and 2 child classes implement these 2 types of functionality.

## impurities
* These now take instance weights.

## Node
* Some vals changed to vars.
 * This is unfortunately a public API change (DeveloperApi).  This could be avoided by creating a LearningNode struct, but would be awkward.

## RandomForestSuite
Please let me know if there are missing tests!

## BaggedPoint
This wraps TreePoint and holds bootstrap weights/counts.

# Design decisions

* BaggedPoint: BaggedPoint is separate from TreePoint since it may be useful for other bagging algorithms later on.

* RandomForest public API: What options should be easily supported by the train* methods?  Should ALL options be in the Java-friendly constructors?  Should there be a constructor taking Strategy?

* Feature subsampling options: What options should be supported?  scikit-learn supports the same options, except for "onethird."  One option would be to allow users to specific fractions ("0.1"): the current options could be supported, and any unrecognized values would be parsed as Doubles in [0,1].

* Splits and bins are computed before bootstrapping, so all trees use the same discretization.

* One queue, instead of one queue per tree.

CC: @mengxr @manishamde @codedeft @chouqin  Please let me know if you have suggestions---thanks!

`def` -> `override val` and remove doc.