Decrypt S3 files

I saw that you would welcome a pull request such as this on the chef cookbook forums. I developed the decryption technique while using a s3 method that used the aws-sdk but this cookbook allows to do without the sdk so I have integrated (and improved!) our decryption feature.  Might as well share it.

This provides a method to decrypt S3 files on the fly.  Files that are sensitive can be encrypted using the AES-256-CBC cipher by a 256bit (32 character) key that is kept secret.

The pull request provides utilities in the bin directory for facilitating encryption and decryption of files prior to putting them to S3 and a utility to generate a valid SHA256 key.