Use `_py2java` and `_java2py` to convert Python model to/from Java model. yinxusen

Author: Xiangrui Meng <meng@databricks.com>

Closes #5049 from mengxr/SPARK-6226-mengxr and squashes the following commits:

570ba81 [Xiangrui Meng] fix python style
b10b911 [Xiangrui Meng] add save/load in PySpark's KMeansModel