Using aws-sdk to generate parameters for direct upload

[janko]

It seems that a lot of carrierwave_direct's logic is generating query parameters for direct S3 upload. However, the aws-sdk gem already has this functionality. Also, carrierwave_direct's logic is Rails/HTML-specific, while aws-sdk generates these parameters as a Ruby hash, so you can use it either to generate HTML fields or to return a JSON response. Maybe carrierwave_direct could just leverage aws-sdk for generating these presign parameters?