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
This `"s"` prefix is just a legacy artifact from Shopify, it shouldn't leak in here. I would make this method return `@cloud.url` by default, and Shopify can override it in its subclass.