Message Dispatcher: Add list of default message handlers for non-matching msg types

* When receiving a message type that is not recognized, the `MessageDispatcher` would previously throw an exception stating that it cannot handle this message.
* This change adds a list of `defaultMessageHandlers`, that will get called with such messages.
* This can be useful for a client that want to be able to handle lots of message types without hardcoding the actual message types.
Merge pull request #48 from iZettle/messagedispatcher-add-default-handler

MessageDispatcher: Add list of default message handlers for non-matching msg types