2.2.0

• Add on_state() handler to states
• New run_machine() method to invoke machine execution (includes a
  check_timed_transitions() call)
• New timed_switchoff.ino example sketch to ilustrate new on_state()
  and run_machine() funcionality
• Corrections:

• make_transition() correctly initialices timed transitions start
  milliseconds (make_transition() is now a fsm method)
• Initial state on_enter() handler is now correctly executed on fsm
  first run
• Removed Serial.println(now); trace in Fsm.cpp
• Correct initialization of m_num_timed_transitions