Base class for queue/exchange declaration responses

It would be nice to have a base class for all the responses reactive-rabbit gives on declarations and bindings. This would allow to have a resonable type when doing Futures manipulation on multiple declarations.

```
val declarations: List[Future[Method]] = 
      List(
        connection.exchangeDeclare("exchange1"), 
        connection.exchangeDeclare("exchange2"), 
        connection.queueDeclare("queue1"), 
        connection.queueDeclare("queue2"))
```

otherwise the type will be `List[Future[Any]]` which is kind of uninformative.