Add a panel to the Resque web-ui

I've added a Metrics tab to the Resque web-ui and made it only load if required in the Gemfile.

![Screenshot of web-ui in action](http://f.cl.ly/items/3Z2P2g1u3R0001081I44/Screen%20Shot%202012-05-05%20at%2010.26.44.png)