Backends and queue depths

This PR adds two things:

* out of the box support for reporting metrics to statsd
* a way to report queue depth metrics

Pretty happy with how they are working out, but... two things occurred to me as I was wrapping this up:

* probably should have split out two different features
* the implementation might have been easier to just use the existing metrics hook, rather than adding in this concept of backends.

Would love feedback if someone wants to try this out in their apps. Here's a snippet for `Gemfile`:

```
gem 'resque-metrics', git: 'https://github.com/technicalpickles/resque-metrics.git', branch: 'backends-and-queue-depths'
```

The README has notes on adding the backend.