add support for users and policies

[rgl]

it would be nice to be able to have multiple users and related policies.

for example, to have a read-only user, one could use the following policy:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "ReadOnly",
      "Effect": "Allow",
      "Action": [
        "s3:ListBucket",
        "s3:GetObject"
      ],
      "Resource": [
        "arn:aws:s3:::*"
      ],
      "Condition": {}
    }
  ]
}