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 was kind of trying to satisfy both worlds -- one where someone provides an S3 connection/bucket and one where they just provide the auth and maybe some options and we use that. 

I think i'm not understanding what you mean by passing the connection via options, something like:

```ruby
cloud = RemoteS3Cloud.new(directory , 'testing/assets/files' )
bucket = cloud.buckets[:tmp]
bucket.write(key, data, s3_bucket: foo)
```

I admit i'm not really a fan of that.