Implement wrappers for unbind and unsubscribe methods on amqp

This pull request adds two additional methods to Bus, and adds one additional value to Bus instances. The additional value is a _consumerTag. The two methods are _unsubscribe() and unsubscribe(). unsubscribe() calls the amqp unbind method on a given topic and exchange. It is the opposite of the already existing subscribe() method on Bus. _unsubscribe() calls the amqp unsubscribe method from the internal queue of the Bus. It is the opposite of the already existing _subscribe() method. The additional value of _consumerTag is now set by _subscribe() in the q.subscribe() callback. This is to maintain the reference necessary to use the unsubscribe method from amqp.