Totals for shuffle data and CPU time

This shows totals for shuffle data and CPU time (including active tasks) on the homepage overview and Stage pages. It should fix [SPARK-807](https://spark-project.atlassian.net/browse/SPARK-807).
![stages](https://f.cloud.github.com/assets/4754931/859440/a4763764-f57e-11e2-97b5-91e0a30ff199.png)
![overview](https://f.cloud.github.com/assets/4754931/859439/a4771b2a-f57e-11e2-893d-530b0530988d.png)