Updated shieldtest and spitftbitmap to work with Mega, Leonardo and Due ...

...using software SD library at https://github.com/adafruit/SD

Tested Adafruit 1.8" shield with:
* Uno using hardware SD library
* Leonardo using software SD library
* Due using software SD library
So long as the correct SD library is in place, the code works unmodified across all three devices
Incorporated soft_spitftbitmap into spitftbitmap and deleted
Renamed *.pde to *.ino