[WIP] [SPARK-2377] Python API for Streaming

This patch bring Python API for Streaming, WIP.

This patch is based on work from @giwa
This change is not necessary anymore because PairwiseRDD is not called in PyhotnDStream. I think just 'private' is enough.

```
-private class PairwiseRDD(prev: RDD[Array[Byte]]) extends
+private[spark] class PairwiseRDD(prev: RDD[Array[Byte]]) extends  # This is not needed any more.
```