SPARK-1407 drain event queue before stopping event logger

Author: Kan Zhang <kzhang@apache.org>

Closes #366 from kanzhang/SPARK-1407 and squashes the following commits:

cd0629f [Kan Zhang] code refactoring and adding test
b073ee6 [Kan Zhang] SPARK-1407 drain event queue before stopping event logger
[SPARK-1357 (fix)] remove empty line after :: DeveloperApi/Experimental ::

Remove empty line after :: DeveloperApi/Experimental :: in comments to make the original doc show up in the preview of the generated html docs. Thanks @andrewor14 !

Author: Xiangrui Meng <meng@databricks.com>

Closes #373 from mengxr/api and squashes the following commits:

9c35bdc [Xiangrui Meng] remove the empty line after :: DeveloperApi/Experimental ::
Merge branch 'master' of https://github.com/apache/spark into SPARK-1413
review commit