Add support for Adafruit Feather M0 (#59)

Support new Cortex M0 boards:

• Adafruit Feather M0 basic proto https://learn.adafruit.com/adafruit-feather-m0-basic-proto/
• Adafruit Feather M0 Bluefruit https://learn.adafruit.com/adafruit-feather-m0-bluefruit-le/overview
• (untested) Arduino Zero https://www.adafruit.com/products/2843

Specs

• ATSAMD21G18 ARM Cortex M0 @ 48 MHz
• 256K FLASH, 32K of RAM, no EEPROM
• 3.3V

pin allocation is different for Feather due to different pinout.
Infrared remote not supported