Add delivery_tag as a read-only property of rabbitpy.Message

Makes it easier/quicker to ack/nack messages, can also use as a message counter ;-)
Add Queue.__iter__, move the once private attributes to non-private ones

- Queue._arguments to Queue.arguments
- Queue._auto_delete to Queue.auto_delete
- Queue._dlx to Queue.dead_letter_exchange
- Queue._dlr to Queue.dead_letter_routing_key
- Queue._durable to Queue.durable
- Queue._exclusive to Queue.exclusive
- Queue._expires to Queue.expires
- Queue._max_length to Queue.max_length
- Queue._message_ttl to Queue.message_ttl
- Move attribute validation to Queue.__setattr__
Add debugging line to help track down issues