[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
It's not common to use file name which starts with underscore, except that it has conflict, so I'd would like to keep `common`, it make the import looks better.