Made Servo library output frequency no longer constant

The old "REFRESH_INTERVAL" has been changed to be settable
 for  better compatability with digital servos and ESC's

The defualt value of 20000 microseconds (50Hz) is maintained,
 and a minimum of 3000 microseconds (333Hz) is imposed.

3000 microsecends was chosen because it allows for
 MINIMUM_PULSE_WIDTH of time low after
 MAXIMUM_PULSE_WIDTH of time high, rounded up by 56 us