[SPARK-16266][SQL][STREAING] Moved DataStreamReader/Writer from pyspark.sql to pyspark.sql.streaming

[tdas]

What changes were proposed in this pull request?

• Moved DataStreamReader/Writer from pyspark.sql to pyspark.sql.streaming to make them consistent with scala packaging
• Exposed the necessary classes in sql.streaming package so that they appear in the docs
• Added pyspark.sql.streaming module to the docs

How was this patch tested?

• updated unit tests.
• generated docs for testing visibility of pyspark.sql.streaming classes.

[tdas]

@zsxwing Can you take a look?

[SparkQA]

Test build #61417 has finished for PR 13955 at commit 94d6019.

• This patch fails Python style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #61419 has finished for PR 13955 at commit 8269e4b.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #61420 has finished for PR 13955 at commit 7dc42d9.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #61431 has finished for PR 13955 at commit cb517c1.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • public final class JavaStructuredNetworkWordCount

[rxin]

can we create a separate file? this file is already very long.

[tdas]

then probably we have to create directory streaming and move all the code in streaming.py into separate files. I wanted to minimize the changes. we can do the bookkeeping later. what do you think?

[SparkQA]

Test build #61432 has finished for PR 13955 at commit e8f4f1d.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #61439 has finished for PR 13955 at commit 50626bd.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[zsxwing]

LGTM. Merging to master and 2.0. Thanks!