[SPARK-2377] Python API for Streaming

This patch brings Python API for Streaming.

This patch is based on work from @giwa
In `DStream.union`, we check the input to verify that the streams have the same slide duration (https://github.com/apache/spark/pull/2538/files#diff-6c576c52abc0624ccb6a2f45828dc6a7R326).  Can we do the same sort of input validation here?