[SPARK-4016] Allow user to show/hide UI metrics.

This commit adds a set of checkboxes to the stage detail
page that the user can use to show additional task metrics,
including the GC time, result serialization time, result fetch
time, and scheduler delay.  All of these metrics are now
hidden by default.  This allows advanced users to look at more
detailed metrics, without distracting the average user.

This change also cleans up the stage detail page so that metrics
are shown in the same order in the summary table as in the task table,
and updates the metrics in both tables such that they contain the same
set of metrics.

The ability to remember a user's preferences for which metrics
should be shown has been filed as SPARK-4024.

Here's what the stage detail page looks like by default:
![image](https://cloud.githubusercontent.com/assets/1108612/4744322/3ebe319e-5a2f-11e4-891f-c792be79caa2.png)


and once a user clicks "Show additional metrics" (note that all the metrics get checked by default):
![image](https://cloud.githubusercontent.com/assets/1108612/4744332/51e5abda-5a2f-11e4-8994-d0d3705ee05d.png)


cc @shivaram @andrewor14
Can you add a // comment here to explain what "classes" means?