ec2: add DescribeAvailabilityZones API support

This commits adds support for the following API endpoint:

http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeAvailabilityZones.html

This API endpoint is not available. It's available in launchpad.net/goamz so I copied them here. Tests are working fine without any problem (I've included them too).