Adding support for EBS Volume pricing including pricing PIOPS

[bbrummer]

This is my first ever pull request...bear with me I'm not sure how all this works yet. ;)

I've added support I needed for pricing EBS volumes to get a more complete picture of EC2 pricing. I did my best to keep the code design and style matching the project's current patterns. Test harness and usage updates included. I've click signed the Lyft agreement.

Although I did consider breaking out EBS into its own object, ultimately I mixed it into the EC2Offer object as EBS is part of the same AmazonEC2 service and given the current design wouldn't make sense to break out. In the long term I'm not sure how well that will scale given how much AWS throws into the EC2 bucket (VPC, ELB, etc), but for now it works well enough.