Merge pull request #128 from shimingfei/joblogger-doc

add javadoc to JobLogger, and some small fix

against Spark-941

add javadoc to JobLogger, output more info for RDD, modify recordStageDepGraph to avoid output duplicate stage dependency information

(cherry picked from commit 518cf22eb2436d019e4f7087a38080ad4a20df58)
Signed-off-by: Reynold Xin <rxin@apache.org>