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

This patch bring Python API for Streaming, WIP.

TODO:
updateStateByKey()
windowXXX()

This patch is based on work from @giwa
Based on your changes, this line should be
```
sc = SparkContext(appName="PythonStreamingWordCount")
ssc = StreamingContext(sc, Seconds(1))
```