Granting an IAM user access to a single bucket is really easy:

1. Create a User in IAM via the Amazon Web Console
2. Assign that user the following policies via: Settings > Attach User Policy > Custom Policy

{
  "Statement": [
    {
      "Action": "s3:*",
      "Effect": "Allow",
      "Resource": [
        "arn:aws:s3:::bucketname",
        "arn:aws:s3:::bucketname/*"
      ]
    }
  ]
}

{
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "s3:ListAllMyBuckets",
      "Resource": "arn:aws:s3:::*"
    }
  ]
}