[SPARK-5133] [ML] Added featureImportance to RandomForestClassifier and Regressor

Added featureImportance to RandomForestClassifier and Regressor.

This follows the scikit-learn implementation here: [https://github.com/scikit-learn/scikit-learn/blob/a95203b249c1cf392f86d001ad999e29b2392739/sklearn/tree/_tree.pyx#L3341]

CC: yanboliang  Would you mind taking a look?  Thanks!

Author: Joseph K. Bradley <joseph@databricks.com>
Author: Feynman Liang <fliang@databricks.com>

Closes #7838 from jkbradley/dt-feature-importance and squashes the following commits:

72a167a [Joseph K. Bradley] fixed unit test
86cea5f [Joseph K. Bradley] Modified RF featuresImportances to return Vector instead of Map
5aa74f0 [Joseph K. Bradley] finally fixed unit test for real
33df5db [Joseph K. Bradley] fix unit test
42a2d3b [Joseph K. Bradley] fix unit test
fe94e72 [Joseph K. Bradley] modified feature importance unit tests
cc693ee [Feynman Liang] Add classifier tests
79a6f87 [Feynman Liang] Compare dense vectors in test
21d01fc [Feynman Liang] Added failing SKLearn test
ac0b254 [Joseph K. Bradley] Added featureImportance to RandomForestClassifier/Regressor.  Need to add unit tests