Add access_token support

Hi Joshua!

This small PR adds [Amazon Security Token](http://docs.aws.amazon.com/STS/latest/UsingSTS/STSUseCases.html) support. This is used among others by the [IAM role credentials](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html).

Thanks for this great cookbook by the way :smile: