Make microseconds_to_angle() consistent with simpleio.Servo()

- simpleio.Servo() allows for the fine tuning of pulse length via
min_pulse and max_pulse but microseconds_to_angle() formerly
ignored those values and used hard coded ones.
- That would lead to an inaccurate conversion if fine tuning had
been used.
- This patch makes use of self.min_pulse and self.max_pulse in
order to make correct use of any fine tuning specified during
object instantiation.

Author: ScottDWebster

simpleio.py

@@ -220,7 +220,7 @@ def angle(self, degrees):
 
     def microseconds_to_angle(self, us): #pylint: disable-msg=no-self-use, invalid-name
         """Converts microseconds to a degree value"""
-        return map_range(us, 500, 2500, 0, 180)
+        return map_range(us, self.min_pulse * 1000, self.max_pulse * 1000, 0, 180)
 
     def deinit(self):
         """Detaches servo object from pin, frees pin"""