Added RDS provider.  

Example Cluster Definition : 
```
Ironfan.cluster 'rds' do
  environment           :_default
  cloud(:rds).security_group(:tests).authorize_ip_range "127.0.0.1/32"
  cloud(:rds).security_group(:tests2).authorize_ec2_group "123456789/systemwide"

  facet :mysql do
    cloud(:rds) do
      password              "abc123"
      username              "root"
      flavor                "db.m1.small"
      size                  5
      port                  3601
    end
  end
end
```