[CHEF-3611] Add ability to download from private AWS S3 buckets

http://tickets.opscode.com/browse/CHEF-3611

Introduce new URL scheme s3://<bucket>/<key> for :recipe_url
that will trigger signing mechanism and convert link to https://
scheme. One could use s3:// or any other valid S3 endpoint,
like s3-eu-east-1://

Required configuration variables are:
'aws_access_key_id' and 'aws_secret_access_key' - your credentials
for secured S3 bucket.
should probably mention in the comment a bit more explicitly that this is only used for chef-solo downloading of recipes, and that it is not the right config attributes for knife bootstrap, etc.