Starting a turbine

I made a recent change to the turbine code that allows you to abort a start properly. With this change the default FlightGear key binding for the 's' key won't work for turbine engines. I'll be adding a new binding definition for the 's' key to all my turbine-powered models. The new binding is a simple toggle, which sets the starter valve to true (open) or false (closed).

The procedure for starting the turbines is: 1) make sure the cutoff property is true (it should be by default), 2) set the starter property to true, 3) when N2 reaches at least 15% place the cutoff property to false. The turbine will accelerate to idle and the starter will automatically set itself to false.