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
I added it thinking it would give people a concrete place to know what they had to implement if they  were making their own cloud that had s3. I can remove it and just leave it implicit.