[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

Author: Xiangrui Meng <meng@databricks.com>

Closes #670 from mengxr/predict-javardd and squashes the following commits:

b77ccd8 [Xiangrui Meng] Merge branch 'master' into predict-javardd
43caac9 [Xiangrui Meng] add predict(JavaRDD) to RegressionModel, ClassificationModel, and KMeans
(cherry picked from commit d52761d67f42ad4d2ff02d96f0675fb3ab709f38)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>