Metrics Endpoint (ready for review)

Add a metrics endpoint to otter.

This will allow otter to provide metrics to a graphite instance. The resource should be accessible from **/v1.0/metrics**. I think having this at **/metrics** would be a better solution as it should not be part of the public API.

Ideas:
 * [x] This should live as it's own service apart from otter. That way any issue with how this behaves will not affect the normal usage of otter.
 * [x] It is currently attached to the *CassScalingGroupCollection* class, and should have its own interface and model representation.

This PR is now ready for review.