add min info gain and min instances per node parameters in decision tree
separate calculation of predict of node from calculation of info gain
fix bug
fix style
fix style
add comments
fix bug
add api docs
Merge branch 'master' of https://github.com/apache/spark into dt-preprune

Conflicts:
	mllib/src/main/scala/org/apache/spark/mllib/tree/configuration/Strategy.scala
minor fix: remove empty lines
fix style
fix bug
fix docs and change minInstancesPerNode to 1
remove `noSplit` and set `Predict` private to tree
Added minInstancesPerNode and minInfoGain parameters to DecisionTreeRunner.scala and to Python API in tree.py
change edge `minInstancesPerNode` to 2 and add one more test
fix typo
fix typo
Merge remote-tracking branch 'chouqin/dt-preprune' into chouqin-dt-preprune
Merge remote-tracking branch 'upstream/master' into chouqin-dt-preprune