[SPARK-2197] [mllib] Java DecisionTree bug fix and easy-of-use

Bug fix: Before, when an RDD was created in Java and passed to DecisionTree.train(), the fake class tag caused problems.
* Fix: DecisionTree: Used new RDD.retag() method to allow passing RDDs from Java.

Other improvements to Decision Trees for easy-of-use with Java:
* impurity classes: Added instance() methods to help with Java interface.
* Strategy: Added Java-friendly constructor
--> Note: I removed quantileCalculationStrategy from the Java-friendly constructor since (a) it is a special class and (b) there is only 1 option currently.  I suspect we will redo the API before the other options are included.

CC: @mengxr
`JavaConversions` -> `JavaConverters`