Start adding support for the EC2 API

This PR contains a start to support for the EC2 API. At the moment it's just some support code in `Aws.Ec2.Core` and support for the `DescribeRegions` command, but it does work (as demonstrated with the included `Examples/Ec2.hs` program).

Most of the details in Core.hs are copy'n'pasted from existing modules, so it'd be good if someone could take a look at them and suggest possible improvements before I get more commands implemented.