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
all these checks to see if `name` is `'load_aws'` is a code smell, suggesting that the `load_aws` behaviour should be handled in a different way.