Add support for separate config and credential INI files

Newer versions of the [AWS CLI Tools](http://docs.aws.amazon.com/cli/latest/reference/configure/index.html) use separate `config` and `credentials` files.  This PR adds support for the new style while keeping backwards compatibility with older unified files.
I guess I just have a personal hatred of heredoc's borking indentation.  I'll switch it around if that's what you'd prefer though.