Merge pull request #90 from pwendell/master

SPARK-940: Do not directly pass Stage objects to SparkListener.

This patch updates the SparkListener interface to pass StageInfo objects rather than directly pass spark Stages. The reason for this patch is explained in detail in SPARK-940.
(cherry picked from commit c404adb9d2238644415fd93270871f5a8a9ce455)

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