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
Yeah i guess people could want different buckets for each instance of S3Bucket. Not a huge fan of the bucket knowing about it's cloud