#3 model file polling

[Meinte Boersma]

Once we start to implement real modularization, we cannot rely on polling the model file for each request -which is not extremely performant anyway.
So, implement/hook up a poller which polls a model directory and implement a listener strategy so the model loader can be signalled in case of a re-load. This is also appropriate to implement, e.g., model updates from source control.