Added in 3 new elb metrics

This will add 3 new metrics for the ELB cloudwatch metrics-plugins.

* **Backend Connection Errors:** the number of errors encountered by the load balancer while attempting to connect to your application.

* **Surge Queue Length:** the number of requests queued by the load balancer, awaiting a back-end instance to accept connections and process the request.

* **Spillovers:** the number of rejected requests due to a full surge queue.

**NOTE** You should delete the ELB dashboard and restart the `cloudwatch.rb` application for the new metrics to get created on the dashboard.