S3 bucket

@jamesmacaulay @bouk /cc @Shopify/admin 

Add S3 bucket to asset_cloud

Does what's in https://github.com/Shopify/asset_cloud/pull/6 without https://github.com/Shopify/asset_cloud/pull/5
Makes sense -- done. 

I'm not sure without this these methods make a tonne of sense on their own, otoh I'm not sure i want to take on responsibility right now, of updating the cloud implementations to do this for every request.