[digitalocean|compute] Add created_at timestamp as attribute of server

I recently switched an app from EC2 over to Digital Ocean and one of the differences in the API was that EC2 instances returned a `created_at` field on the `Server`, whereas Digital Ocean did not. After digging into the raw response Digital Ocean sends back, it turns out all we need to do is add the field as an attribute of the `Server` and it all just works.

Example:
```ruby
2.0.0p247 :002 > fog.servers.get(567)
 =>   <Fog::Compute::DigitalOcean::Server
    id=567,
    name="name",
    state="active",
    image_id=12345678,
    region_id=4,
    flavor_id=66,
    public_ip_address="1.2.3.4",
    private_ip_address=nil,
    backups_active=false,
    created_at="2013-12-06T13:54:06Z"
  > 
```