Merge branch 'zero-core'

cores/arduino/USB/samd21_host.c

cores/arduino/Uart.cpp

cores/arduino/WInterrupts.c

cores/arduino/avr/dtostrf.c

cores/arduino/avr/dtostrf.h

cores/arduino/avr/interrupt.h

cores/arduino/avr/pgmspace.h

cores/arduino/wiring.c

cores/arduino/wiring_analog.c

cores/arduino/wiring_constants.h

cores/arduino/wiring_digital.c

cores/arduino/wiring_digital.h

cores/arduino/wiring_private.c

cores/arduino/wiring_private.h

libraries/SPI/SPI.cpp

libraries/SPI/SPI.h

libraries/Wire/Wire.cpp

libraries/Wire/Wire.h