Added IAM Instance Profile support

Adds IAM Instance Profile (IIP) support to packer AWS/ebs.  Example config:

{
  "builders": [{
    "type": "amazon-ebs",
    "region": "us-west-1",
    "source_ami": "ami-12345678",
    "iam_instance_profile": "my_iam_role",
    "instance_type": "t1.micro",
    "ssh_username": "ec2-user",
    "ami_name": "packer-test"
  }]
}

Also requires modification to michellh/amz/ec2/ec2.go