thermo demo only for Arduino Mega

Add a frame buffer graphic interface which subclasses
Adafruit_GFX library and is suitable for AVR MCU that has at least
8 kBytes of SRAM such as the Arduino Mega.

Designs using ATmega1280 or ATmega2560 should also work.

download and install Adafruit_GFX under in:
  Sketches/libraries/Adafruit_GFX

Signed-off-by: Christopher Hall <christopherhall.hsw@gmail.com>