Serializer configuration support

Serailizers are no longer hardcoded into the `Serialization` class.
We are now using the correct config sections:

```javascript
akka.actor {
    serializers {
        acme = "Acme.Inc.MySerialier"
	}

    serialization-bindings {
      "Acme.My.MessageType" = acme 
    }
}
```
(do note that serializers from fallback will also be used, the same sections are used for the entire fallback chain)

Allows #308 to apply e.g. DataTableSerializer or such to a given message type
Yes, but those values also existed in pigeon conf, which had priority over it since remote conf was appended as a fallback.
Thus, those values was not actually used in practice, even if they are the correct ones.

So specs should be updated accordingly.