[SPARK-5597][MLLIB] save/load for decision trees and emsembles

This is based on #4444 from jkbradley with the following changes:

1. Node schema updated to
   ~~~
treeId: int
nodeId: Int
predict/
       |- predict: Double
       |- prob: Double
impurity: Double
isLeaf: Boolean
split/
     |- feature: Int
     |- threshold: Double
     |- featureType: Int
     |- categories: Array[Double]
leftNodeId: Integer
rightNodeId: Integer
infoGain: Double
~~~

2. Some refactor of the implementation.

Closes #4444.

Author: Joseph K. Bradley <joseph@databricks.com>
Author: Xiangrui Meng <meng@databricks.com>

Closes #4493 from mengxr/SPARK-5597 and squashes the following commits:

75e3bb6 [Xiangrui Meng] fix style
2b0033d [Xiangrui Meng] update tree export schema and refactor the implementation
45873a2 [Joseph K. Bradley] org imports
1d4c264 [Joseph K. Bradley] Added save/load for tree ensembles
dcdbf85 [Joseph K. Bradley] added save/load for decision tree but need to generalize it to ensembles