implement basic dead letter exchange behaviour

[aiomaster]

I tried to implement a basic handling of dead-letter-exchanges.
If a message gets nacked or rejected it could be delivered in a dead letter exchange if specified in the arguments parameter of the queue options.
See here for details:
http://rubybunny.info/articles/extensions.html#dead_letter_exchange_dlx

I would be glad to get some feedback on this PR cause I don't know if this is a good idea and if the code is good enough.

[coveralls]

Coverage remained the same at 100.0% when pulling 6729e0d on webit-de:master into 468fe86 on arempe93:master.

[coveralls]

Coverage remained the same at 100.0% when pulling b53c0fd on webit-de:master into 468fe86 on arempe93:master.

[arempe93]

hey @aiomaster - thanks for the PR! i'll try to take a look and merge it soon 👍

[aiomaster]

@arempe93 This would be great! I just tested it for direct exchanges with queues having a dead letter queue and the only way to get dead letters with bunny mock for now is by rejecting the message.
It would need more effort to simulate a dead lettering because of message timeout.
The last dead letter reason is a full queue, but this is also difficult with bunny-mock, cause you have to define what "full" means.