Support multiple rooms

Fixes #49 
I think this part will be better this way:

    if (params == null) {
        params = new WebhookParameters(configuration.getRoomId());
    } else {
        if (params.getRoom() == null) {
            params.setRoom(configuration.getRoomId());
        }
        // ^^^ or, you could move this to a well-named method inside WebhookParameters
    }

This has the advantages:
- no more questionable `init` method
  + no more questions about the timing of the `init` call and enforcing it
  + no more questions about what `init` does, and the logic it uses to set the room id. It's all clear here
- decouple `WebhookParameters` from `Configuration`: it's usually not a good thing to pass a complex object to a method just so that it can extract a single field from it. It's better to give it the single field directly. In other words, don't pass a method more than it really needs.