add puma_metrics

This adds metrics for the [Puma webserver](http://puma.io/).

For the plugin to work, the user must add `--control auto --state "/tmp/puma.state"` to the start command of the Puma instance.

Example run command:

    puma --control auto --state "/tmp/puma.state" -p $PORT config.ru

Example output:

    server.example.com.puma.backlog 0 1412601561
    server.example.com.puma.running 1 1412601561

Thanks!