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.
Fun fact (doesn't block this checkin but if you want ...) you can replace this with:

```ruby
        ini.write(<<-EOM)
[personal]
aws_access_key_id = AKIAPERSONALKEY
aws_secret_access_key = personalsecretaccesskey
[work_iam]
aws_access_key_id = AKIAWORKIAMKEY
aws_secret_access_key = workiamsecretaccesskey
        EOM