First pass at pulling metrics from AWS ELBs

I've put together a first pass at pulling ELB metrics from AWS CloudWatch.

#9 talks about adding cloudwatch support, which I thought about doing, but I thought it might turn into a sprawling tool - and maybe it's preferred to target the individual services?

This will connect to cloudwatch and pull the metrics for all of the ELB as listed in the [CloudWatch Developer Guide](http://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/CW_Support_For_AWS.html#elb-metricscollected).

You can report metrics for different load balancers, the event host is setup to be the load balancer name (which makes sense in my org, but it's something I can add code into so it can be overridden if you think that's a bit too prescriptive).

It supports using a fog credentials file, or passing in your api keys on the command line.

By default it will only show the aggregated metrics from all of the AZs in a region, however you can pass in a list of AZs to get the different metrics reported.