Aws sdk v2

* Added s3 storage implementation using the aws-sdk v2.

There are still a few things that need fixing (see pending specs):
* Flaky specs. Is this due to the v2 stubbing?
* s3_host_name - should it be replaced s3_region
* Figure out credential_provider spec.
* Some spec tests SlowDown exception which where a v1 concept.

I don't see any reason it wouldn't be. We use it for building hostname-style URLs. Are these invalid in the new API?