Create button_controled_continuous_rotating_servo

// A sketch to control a continuous servo using 
// 1) two buttons, one incriments a state counter for rotate/stop, the other for direction of rotation
// 2) one analog sensor mapped to allow normal "zero to max" speed control regarless of direction
// 3) one three wire continuous rotating servo:  stop=90 : max clockwise=180 : max anticlockwise=0