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).
Merge pull request #122 from fatih/master

ec2: add DescribeAvailabilityZones API support