Adds a worker list view to the visualizer

This gives you a quick view into what each worker is up to. Since adding it to my own fork last week, it has become the most useful page on the visualizer for me.

![image](https://cloud.githubusercontent.com/assets/2091885/4550970/fba9d8aa-4e6b-11e4-9f07-74d170f84501.png)

Note: the screenshot includes #488 for number of worker processes.