[SPARK-2377] Python API for Streaming

This patch brings Python API for Streaming.

This patch is based on work from @giwa
I added this function as public method at first. That's why davices left this function in DStream. As far as my understanding, saveAsPickleFile is saveAsObjectFile in Scala/Java. If so, it would be good to be a public method.

Python saveAsPickleFile doc string
```
def saveAsPickleFile(self, path, batchSize=10): 
         """ 
          Save this RDD as a SequenceFile of serialized objects. The serializer 
          used is L{pyspark.serializers.PickleSerializer}, default batch size 
          is 10. 
         """
```


Scala
```
def saveAsObjectFile(path: String): Unit
Save this RDD as a SequenceFile of serialized objects.
```