Add Support for Rackspace CDN enabled Containers

This changes adds the ability to mark containers as CDN enabled.

```Javascript
client.getContainer('my-container', function(err, container) {
  if (err) {
    // handle error
  }

  container.updateCdn({ enabled: true }, function(err, container) {
    if (err) {
      // handle error
    }
 
    console.log(container.cdnUri);
  });
});
```

By default, `client.getContainers` does not load CDN details, but you can optionally get that data, although this results in an `1+n` queries outcome as each containers details is loaded individually:

```Javascript
client.getContainers({
    loadCDNAttributes: true
}, function (err, containers) {
  // do stuff
});
```

### CDN Container Attributes

* cdnEnabled
* ttl
* logRetention
* cdnUri
* cdnSslUri
* cdnStreamingUri
* cdniOSUri