[SPARK-4124] [MLlib] [PySpark] simplify serialization in MLlib Python API

Create several helper functions to call MLlib Java API, convert the arguments to Java type and convert return value to Python object automatically, this simplify serialization in MLlib Python API very much.

After this, the MLlib Python API does not need to deal with serialization details anymore, it's easier to add new API.

cc mengxr

Author: Davies Liu <davies@databricks.com>

Closes #2995 from davies/cleanup and squashes the following commits:

8fa6ec6 [Davies Liu] address comments
16b85a0 [Davies Liu] Merge branch 'master' of github.com:apache/spark into cleanup
43743e5 [Davies Liu] bugfix
731331f [Davies Liu] simplify serialization in MLlib Python API