Initial state should be entered

When I was implementing some enemy AI with the `StateMachine`, I stumbled over a "feature" that I did not expect at first.

I was assuming that whenever a `State` gets `update()`ed, it has been `enter()`ed before. However in case of the initial state this does not happen. I think it won't harm anyone to enter the initial state as well, and in my case it saved me from some NPEs :)

Maybe we should also add a call to `exit()` in case the current state is not `null` when `setInitialState()` is called. Usually this should not be called several times, but it is public and the API does not deny it. If I'd use the method myself, I'd expect it to `exit()` my current state as well.

@davebaol: What do you think?