[SPARK-2377] Python API for Streaming

This patch brings Python API for Streaming.

This patch is based on work from @giwa
We have a `context` method that returns the StreamingContext and this public `ctx` field that returns the underlying SparkContext.  This seems like it could be confusing, so maybe we should make the `ctx` field private.  On the other hand, RDD has a `ctx` field that returns the SparkContext, so this is consistent with that.