[SPARK-2377] Python API for Streaming

This patch brings Python API for Streaming.

This patch is based on work from @giwa
Wrong doc string. The Scala doc string is 
/**
   * Stop the execution of the streams, with option of ensuring all received data
   * has been processed.
   * @param stopSparkContext Stop the associated SparkContext or not
   * @param stopGracefully Stop gracefully by waiting for the processing of all
   *                       received data to be completed
   */