Refactor for Librato

Everyone,

I've made a few small tweaks to this gem:

* Refactored the log line to make it more Librato-friendly
* Removed timing stuff (as it's now available in `rack-timing`)
* Enabled periodic reporting or per-request reporting with `RACK_QUEUE_METRICS_INTERVAL` in your environment

If you want to try this in your app I'll make it easy for you:

```
gem 'rack-queue-metrics', git: "https://github.com/heroku/rack-queue-metrics.git", branch: "cb-logging"
```

Which lets you have cool graphs like this, from a few `ab` tests on a slow page:

![image](https://www.evernote.com/shard/s1/sh/947d2598-4e12-44c4-8641-01b970367ae9/b1db7fd35a39c61c3891ba6254342502/deep/0/Instrument--Unicorn-Workers-and-Queues---Librato.png)

cc @wuputah