Allow Separate Backend and Broker

This pull request will allow the user to specify a separate broker from their back end. So using Redis for results and rabbitmq for pushing tasks. The Celery Constructor will now take two arrays as parameters the first being connection details for the broker and the second being (optionally) connection details for the back end. A simple array would look like this. 

$brokerConnection = array(
        "host" => "localhost",
        "login" => "gdr",
        "password" => "test",
        "vhost" => "wutka",
        "exchange" => "celery",
        "binding" => "celery",
        "port" => 5672,
        "connector" => "php-amqplib"
);
$c = new Celery($brokerConnection);