#22 Add events on Consumer and RpcServer

Implemented events in consumer class.

Logger is not a consumer dependency anymore.
Callbacks and logger are not required, but compatible with the old configuration.
The result for deliveryCallback is the return value of the last listener. It's possible to set the right priorities or to stop execution in the listener.

I've done some little refactoring and tests.

We need to add a check here. Something like:

    if (!$results->stopped()) {
        throw new RuntimeException('No result found, bla bla bla');
    }