Make it optional to use the RST pin

Without RST can run without 'root' by adding the desired user
to the I2C group (e.g.: adduser pi i2c)

Adafruit_SSD1306/SSD1306.py

@@ -85,7 +85,8 @@ class SSD1306Base(object):
             self._gpio = GPIO.get_platform_gpio()
         # Setup reset pin.
         self._rst = rst
-        self._gpio.setup(self._rst, GPIO.OUT)
+        if not self._rst is None:
+            self._gpio.setup(self._rst, GPIO.OUT)
         # Handle hardware SPI
         if spi is not None:
             self._log.debug('Using hardware SPI')
@@ -150,6 +151,8 @@ class SSD1306Base(object):
 
     def reset(self):
         """Reset the display."""
+        if self._rst is None:
+            return
         # Set reset high for a millisecond.
         self._gpio.set_high(self._rst)
         time.sleep(0.001)