amazon/common: Added AMI CopyImage support

Copy the AMI to various regions.  Example config:

```json
{
  "builders": [{
    "type": "amazon-ebs",
    "access_key": "YOUR KEY HERE",
    "secret_key": "YOUR SECRET KEY HERE",
    "region": "us-east-1",
    "source_ami": "ami-de0d9eb7",
    "instance_type": "t1.micro",
    "ssh_username": "ubuntu",
    "ami_name": "packer-example {{timestamp}}",
    "ami_description": "Packer Example",
    "ami_users": [ "444455556666", "444455556667" ],
    "ami_regions": [ "us-west-1", "us-west-2" ],
    "ami_block_device_mappings":[{
      "device_name": "/dev/sda1",
      "volume_size": 8,
      "delete_on_termination": true
    }],
    "tags": {
      "Name": "Packer Example"
    }
  }]
}
```