[SPARK-18239][SPARKR] Gradient Boosted Tree for R

## What changes were proposed in this pull request?

Gradient Boosted Tree in R.
With a few minor improvements to RandomForest in R.

Since this is relatively isolated I'd like to target this for branch-2.1

## How was this patch tested?

manual tests, unit tests

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #15746 from felixcheung/rgbt.

(cherry picked from commit 55964c15)
Signed-off-by: Felix Cheung <felixcheung@apache.org>