[SPARK-1644] The org.datanucleus:* should not be packaged into spark-assembly-*.jar

Author: witgo <witgo@qq.com>

Closes #688 from witgo/SPARK-1644 and squashes the following commits:

56ad6ac [witgo] review commit
87c03e4 [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1644
6ffa7e4 [witgo] review commit
a597414 [witgo] The org.datanucleus:* should not be packaged into spark-assembly-*.jar
(cherry picked from commit 561510867a1b79beef57acf9df65c9f88481435d)

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