Add prototypes for SPI functions

Added SPI.h and SPI.cpp to libarduino/cores/virtual/
Add libraries folder to libarduino/

Move SPI.h and SPI.cpp to libarduino/libraries/
Rename _pin_type SPI to _SPI

Conflicting with SPIClass object
Add SPI.h path to SPI.cpp

Commented warning causing definition
Add path to SPI folder in ARDUINO_LIB in makefile