[SPARK-2377] Python API for Streaming

This patch brings Python API for Streaming.

This patch is based on work from @giwa
This class could also use a docstring:

> This class implements a serializer for `PythonTransformFunction` Java objects.  This is necessary because the Java PythonTransformFunction objects are actually Py4J references to Python objects and thus are not directly serializable.  When Java needs to serialize a PythonTransformFunction, it uses this class to invoke Python, which returns the serialized function as a byte array.

Does this sound okay (feel free to correct / amend if it's not)?