Add `protocol` option to prevent "Mixed content" error in Firefox

If you are on an `https` domain and you try to upload to an s3 bucket under `http`, it will throw an error (at least in Firefox):

https://blog.mozilla.org/tanvi/2013/04/10/mixed-content-blocking-enabled-in-firefox-23/

The error looked like this:

[![https://blog.mozilla.org/security/files/2013/06/WebConsoleMixedContentOutput.jpg](https://blog.mozilla.org/security/files/2013/06/WebConsoleMixedContentOutput.jpg)](https://blog.mozilla.org/security/files/2013/06/WebConsoleMixedContentOutput.jpg)
Merge pull request #6 from viatropos/patch-1

Add `protocol` option to prevent "Mixed content" error in Firefox