Added passing UserData to goamz.

Simple addition. Allow the configuration for EC2 to pass UserData from the configuration file into the instance.

Example:

<pre>
{
  "builders": [{
    "type": "amazon-instance",
    "access_key": "xxx",
    "secret_key": "xxx",
    "region": "us-east-1",
    "source_ami": "ami-1234567",
    "instance_type": "c1.xlarge",
    "user_data": "#cloud-config\nhostname: system2",
    "ssh_username": "ubuntu",
    "account_id": "xxxx",
    ...
  }]
</pre>