[SPARK-5326] Show fetch wait time as optional metric in the UI

With this change, here's what the UI looks like:

![image](https://cloud.githubusercontent.com/assets/1108612/5809994/1ec8a904-9ff4-11e4-8f24-6a59a1a858f7.png)

If you want to locally test this, you need to spin up multiple executors, because the shuffle read metrics are only shown for data read remotely.

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #4110 from kayousterhout/SPARK-5326 and squashes the following commits:

610051e [Kay Ousterhout] Josh style comments
5feaa28 [Kay Ousterhout] What is the difference here??
aa129cb [Kay Ousterhout] Removed inadvertent change
721c742 [Kay Ousterhout] Improved tooltip
f3a7111 [Kay Ousterhout] Style fix
679b4e9 [Kay Ousterhout] [SPARK-5326] Show fetch wait time as optional metric in the UI