Hostname in object key

Though this doesn't change the default `s3_object_key_format`, it adds ability to use hostname as part of the S3 object key. This is out of concern for atomicity of S3 object creation from multiple machines. The uploading algorithm right now is:

```ruby
i = 0
begin
  s3path = ...
  i += 1
end while @bucket.objects[s3path].exists?

... # gzip file

@bucket.objects[s3path].write(gz_file) # upload
```

The concern is, during the compression phase another machine could have started the upload. And AFAIK upload overwrites files if they exist.

Note: I can update README if this is fine.