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
Is this globally configuring the AWS library? We really should not do it that way.

My preference would be for an `S3Bucket` provided by the gem to not actually talk to global AWS classes/modules directly, but rather call methods on AWS connection object instances passed in through options. Then you don't have to do any of the `eager_autoload!` stuff either.