Updated SSD1306 code to be compatible with the Arduino DUE

I purchased a 0.96" 128x64 OLED graphic display to use with my Arduino Due only to find out that I couldn't use it! The fixes to get it working were pretty minimal and it didn't require altering the GFX library. 

This is working with SPI on the pins (9-13) as described in the tutorial at http://learn.adafruit.com/monochrome-oled-breakouts, I have not tried using I2c, or SPI on any of the other pins.

![photo copy](https://f.cloud.github.com/assets/1625899/1303306/203c6b04-3150-11e3-8bd5-a0bc771784a0.JPG)