Decouple queueing mechanism from MongoDB

[lstrojny]

To allow using a different queueing system, like e.g. RabbitMQ, the Queue functionality should be decoupled from the actual queueing implementation. To allow as many queue technologies as possible, the requirements should be as simple as possible. A list of things from my knowledge more or less every queue supports:

• Append a message to the queue
• Read a single message
• Mark a single message as processed

[enygma]

Based on the state in this new tag (1.4) you can now inject a Queue object instance into the Manager of your choosing - it just has to extend \Expose\Queue.

https://github.com/enygma/expose/tree/1.4