add aws_read_options and aws_write_options

Hi:

We created aws_read_options and aws_write_options for carrierwave-aws; this allows us to send & receive an encryption_key which enables client-side encryption; see here:

https://ruby.awsblog.com/post/Tx1OFBULXYZNVTH/Using-Client-Side-Encryption-for-S3-in-the-AWS-SDK-for-Ruby

Can you please incorporate this pull request?

Thanks,
Greg & Erik