Added transition_callback

[MartinBroers]

@ryan-summers Thanks for merging !66. I also like the function of a transition callback, so I went straight on it.

In terms of refinement and/or discussion, I am doubting mostly about this right now:

                                              self.context().transition_callback(&in_state, &out_state);
                                              self.state = Some(out_state);

Or should it be like this:

                                              self.state = Some(out_state);
                                              self.context().transition_callback(&in_state, &out_state);

So, call the transition callback after the internal state has been updated?

[ryan-summers]

I don't think it matters too much since the user will have both the input and output state directly in the method call as opposed to via the context.