Allow to optionally not have refractoriness in a model

Relatively simple change to close #125 and make the refractoriness code optional (`NeuronGroup`'s refractory argument has to be set to something else than `False`). Simplifies `PoissonGroup` a bit which no longer needs a state updater just to reset the refractoriness in every time step... I think it's ready to merge as it is.