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
These config properties shouldn't be global. I was thinking one way to do it in a non-global way would be to force the cloud object to implement an `s3_options` method which returns an options hash (then from the `S3Bucket` class, refer to `cloud.s3_options`).