Add Rackspace Queue functionality

I've created the classes to integrate RackSpace Queue functionality, implementing the PHP OpenCloud SDK (https://github.com/rackspace/php-opencloud).

This snippet is all that needs to be added inside 'connections' in app/config/queues.php.
```
'rackspace' => array(
			'driver'    => 'rackspace',
            'username'  => 'your-username',
            'apiKey'    => 'your-api-key',
			'endpoint'  => 'UK',  // supported options: UK or US
			'region'    => 'your-region',  // eg. LON, DFW, ORD
			'urlType'   => 'publicURL', // valid options: publicURL or internalUrl
            'queue'     => 'your-queue-name'
		),
```

Let me know if I need to create a pull request on laravel/laravel to add the above into in app/config/queues.php.