Add ability to use an AWS profile instead of explicit ‘s3_id’ and ‘s3_secret’

[c0state]

This change seeks to add the ability to use the well defined AWS profile credentials to access S3, instead of the previous s3_id and s3_secret, in a backward compatible manner.

You can now create a config.yml as:

profile: <aws_profile_name>
s3_bucket: <my_bucket_name>

where profile is defined in your ~/.aws/config and ~/.aws/credentials files,

instead of:

s3_id: <aws_key_id>
s3_secret: <aws_key_secret>
s3_bucket: <my_bucket_name>