Added support for setting attributes on the AMI

Added a new amazon/common step for modifying ami attributes (ModifyImageAttribute). The main purpose is for authorizing an AMI after it's been created.  This change is contingent on this pull request: https://github.com/mitchellh/goamz/pull/6

Example ebs config:
```json
{
  "type": "amazon-ebs",
  "region": "us-east-1",
  "source_ami": "ami-de0d9eb7",
  "instance_type": "t1.micro",
  "ssh_username": "ubuntu",
  "ami_name": "packer-quick-start {{.CreateTime}}",
  "ami_description": "My Description",
  "ami_users": ["444455556666", "444455556667"]
}
```

Example instance config:
```json
{
  "type": "amazon-instance",
  "region": "us-east-1",
  "source_ami": "ami-de0d9eb7",
  "instance_type": "t1.micro",
  "ssh_username": "ubuntu",
  "ami_name": "packer-quick-start {{.CreateTime}}",
  "ami_description": "My Description",
  "ami_groups": ["all"],
  "ami_product_codes": ["123456789"]
}
```