Threadpool graphes row

I found myself using http request more than often to monitor thread_pool metric, so I thought that would be interesting to add it into bigdesk and that's what I've done. Pretty straightfoware code, but that's how you did it so...

Wanted to share with you, so if you're interested, just have to accept this! :)

![thread_pools](https://f.cloud.github.com/assets/568924/197036/6ddcf752-802f-11e2-85b3-71160a80f90e.png)