LIBCLOUD-470: VPC Elastic IP Support

I tried to make this change without impacting some of the old calls (eg: ex_describe_addresses) using keyword arguments. All of my tests using both the mock data and my own AWS account appear to work without issue for both EC2 and VPC elastic IP addresses. 

If you think it's better to create a class for network addresses such as EC2NetworkAddress I can certainly make that change. 

Lastly, Merry Christmas and enjoy the holiday!