Merge pull request #561 from Qiuzhuang/master. Closes #561.

Kill drivers in postStop() for Worker.

 JIRA SPARK-1068:https://spark-project.atlassian.net/browse/SPARK-1068

Author: Qiuzhuang Lian <Qiuzhuang.Lian@gmail.com>

== Merge branch commits ==

commit 9c19ce63637eee9369edd235979288d3d9fc9105
Author: Qiuzhuang Lian <Qiuzhuang.Lian@gmail.com>
Date:   Sat Feb 8 16:07:39 2014 +0800

    Kill drivers in postStop() for Worker.
     JIRA SPARK-1068:https://spark-project.atlassian.net/browse/SPARK-1068
(cherry picked from commit f0ce736fadbcb7642b6148ad740f4508cd7dcd4d)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>