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
@jamesmacaulay I think i need some direction here.

Any way that I call back to the cloud either via instantiation or call back via buckets `@cloud`
 I have to make either any implementer of `AssetCloud::Base`with s3 buckets be aware that it has to implement some s3 fetching method (I don't like this because It's implicit), or I provide one on base itself (I don't like this because I don't think S3 is a responsibility of `AssetCloud::Base`).