Adds the new C3 instances

Data cobbled together from:
- http://aws.typepad.com/aws/2013/11/a-generation-of-ec2-instances-for-compute-intensive-workloads.html
- http://aws.amazon.com/ec2/pricing/

Thanks for a helpful service! I use it almost every day.
Merge pull request #49 from drob/master

Adds the new C3 instances