Implemented support for gathering credentials from AWS CLI configuration file

Per the question from audience in re:Invent session, implemented using the AWS CLI configuration file credentials. I hesitated to use a gem to parse the Python config file, but I thought it would be better than reinventing the wheel.

See: http://docs.aws.amazon.com/cli/latest/userguide/cli-chap-getting-started.html#cli-installing-credentials