Add support for IAM roles

Currently for S3 integration the module requires we specify an AWS key and secret. This makes sense if your app is not running on EC2 but if your app is running on EC2 the AWS best practices state that you should use an IAM Role when you need one service to access another.

We recently started converting our services to use IAM Roles but then hit a wall when we noticed that the AWS keys are required for mongoose-attachments to work.