AWS SDK v2 + numerous improvements

Upgraded the gem to work with AWS SDK v2, because v1 is deprecated.
Numerous fixes and improvements.

Important notes:

• Now requires AWS_REGION to be able to connect.
• Now supports new env variables to specify availability zone and device name (optional and used for restore only).
• Uses expires_on tag name instead of expires_after tag name on snapshots to mark their expiration. This is done to be more correct/consistent. This rename means you need to remove all you old snapshots manually, because the removal process will not find the old snapshots with expires_after key.

And last but not least, please share your ideas and pull requests to help me improve the gem! Any help is greatly appreciated.