Show app duration

This will show the app duration in the Job UI, fixing [SPARK-843](https://spark-project.atlassian.net/browse/SPARK-843).
![screen shot 2013-08-02 at 1 29 41 pm](https://f.cloud.github.com/assets/4754931/903726/4d327334-fbb2-11e2-9408-0f9b3692e7c0.png)