Added a Fritzing model of the Mega2560 prototype

- only Breadboard view has to be considered
- some items are not truly represented
* DS1302 module is a breakout (DX sku126453)
* Sensors are not the definitive one but for testing purpose
** Differential pressure is a raw MPX5100DP 
** Absolute pressure is a raw MPX5100A
** internal an external temperature sensors are LM35
** voltage sensor is a voltage divider (/2)