Update erltrek_game.erl

* add enterprise_command/1 for giving timer-synchronized command of
  Enterprise actions
* add handle_call(enterprise_command, ...) to process the
  action request, and refuse request if any command is processed ongoing
Update erltrek_event.erl

* Change timer_tasks/1 to timer_tasks/2
  (separating Tick and GameState)
* Comment out old testing code in timer_tasks/2
* Add enterprise_command/2 to process buffered command
  in #enterprise_status.next_command
  (Here you cannot change the Tick value)