Ability to override response_headers in generate_url

[cancan101]

Add the ability to override response_headers in generate_url. This is important if you want to set something like Cache-Control.

[taion]

I don't think that's the right API. For S3, we need to set the expiration time both on the S3 side and on the response side; those should derive from the same value.

[cancan101]

Closing in favor of #19.