Fix #80

Make web socket message processing asynchronous.
I've extended the Akka System Service to add a new method letting dispatching Callable<T>.

Signed-off-by: Clement Escoffier <clement.escoffier@gmail.com>
Add the subprotocols configuration property in the architectype

Signed-off-by: Clement Escoffier <clement.escoffier@gmail.com>