remove workers button on resque ui

Adds a button to clean up old workers. We had dead servers/workers that were still being reported.

It's kind similar to this issue, but a less automatic approach: https://github.com/defunkt/resque/issues/319

![Button](https://f.cloud.github.com/assets/36106/91521/a04876ae-65a3-11e2-96f6-fafb918034b7.png)