Demo for usage of StateMachine

[JeroenSoeters]

Example for the usage of act.StateMachine. This example is inspired by the code lock example from the gen_statem documentation.

We model a code lock as a state machine. This example demonstrates the following features:

• StateMessageHandler allows you do register a message for a specific state. Here for example we register the handleButtonPress handler that receives the ButtonPress message in the open state.
• StateCallHandler similar to StateMessageHandler but represents a synchronous call with a return value. In this example we register a StateCallHandler to query the length of the code.
• StateEnterCallback allows for a callback that is triggered on every state transition. This is useful for orthogonal concerns like logging, auditing etc.
• StateTimeout provides a timer that sends a message to the state machine on timeout. If the state machine transitions to another state before the timeout, the timer is canceled. In this example we set a state timeout for the open state. After 10 seconds in this state the state machine automatically moves to the locked state.
• MessageTimeout is useful for measuring inactivity. We set up a message timeout after pressing each button. After 30 seconds of inactivity the buttons are cleared. A MessageTimeout timer is canceled by any type of message or event.

See ergo-services/ergo#218