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
Yeah, i'm not sure how i feel about this class -- maybe a bit YAGNI. I looked at how some other gems did configuration, my thinking was we'd be putting more of our cloud stuff in here, and it'd be more generally useful in the future.