Moar cloud

@bouk @jamesmacaulay /cc @Shopify/admin 

Adds the S3 interface from shopify to asset_cloud

Dependant on https://github.com/Shopify/asset_cloud/pull/5
here https://github.com/Shopify/asset_cloud/pull/6/files#diff-37b3a967d37d97d8573c5b19df0229d7R83