Place holder resolution in @RabbitListener arguments

This commit allows to use place holder definitions for RabbitListener
attributes, effectively allowing to externalize those settings from
the code.

Issue: AMQP-424
I don't think `exclusive` really needs to be configurable - that will be an up-front design issue.

Priority should be configurable via placeholders; I can see that someone might want to set the priority based on whatever size server the application instance is deployed on.

However, I just noticed a bug - it currently defaults to `-1` and is only applied when positive.

The rabbit docs say:

> Set the x-priority argument in the basic.consume method to an integer value. Consumers which do not specify a value have priority 0. Larger numbers indicate higher priority, and both positive and negative numbers can be used.

http://www.rabbitmq.com/consumer-priority.html