[SPARK-1741][MLLIB] add predict(JavaRDD) to RegressionModel, ClassificationModel, and KMeans

`model.predict` returns a RDD of Scala primitive type (Int/Double), which is recognized as Object in Java. Adding predict(JavaRDD) could make life easier for Java users.

Added tests for KMeans, LinearRegression, and NaiveBayes.

Will update examples after https://github.com/apache/spark/pull/653 gets merged.

cc: @srowen