#75 - Default routingKey

By declaring th routingKey attribut to '' we have a default. The php-amqp lib implements this same default in:
* exchange_bind: https://github.com/videlalvaro/php-amqplib/blob/master/PhpAmqpLib/Channel/AMQPChannel.php#L300

This I expect will solve the #75 issue.
* queue_bind: https://github.com/videlalvaro/php-amqplib/blob/master/PhpAmqpLib/Channel/AMQPChannel.php#L326