Add MachineEvent class for custom lightweight event handling

[ganju001]

I know that some people would like to integrate already existing OO code with Automaton.
For that purpose it would be nice to have some lightweight interface class for events.

1. I know that I can extend and implement Machine class but it is quite heavy for such a light purpose.
2. It is possible to use std::bind to create function from method calls but it is not available in all environments.