add ability to create object without RST pin (default -1)

Adafruit_SSD1306.cpp

@@ -203,7 +203,7 @@ void Adafruit_SSD1306::begin(uint8_t vccstate, uint8_t i2caddr, bool reset) {
 #endif
   }
 
-  if (reset) {
+  if (reset >= 0) {
     // Setup reset pin direction (used by both SPI and I2C)  
     pinMode(rst, OUTPUT);
     digitalWrite(rst, HIGH);