[SPARK-5275] [Streaming] include python source code

Include the python source code into assembly jar.

cc mengxr pwendell

Author: Davies Liu <davies@databricks.com>

Closes #4128 from davies/build_streaming2 and squashes the following commits:

546af4c [Davies Liu] fix indent
48859b2 [Davies Liu] include python source code

streaming/pom.xml

@@ -96,5 +96,13 @@
         </executions>
       </plugin>
     </plugins>
+    <resources>
+      <resource>
+        <directory>../python</directory>
+        <includes>
+          <include>pyspark/streaming/*.py</include>
+        </includes>
+      </resource>
+    </resources>
   </build>
 </project>