Split Ethernet library into it's own NuGet package.
- bring avr directory reference into NuGet
- add avr/macros.h and avr/interrupt.h files
- add necessary `#include <cstring>` into avr/pgmspace.h
Remove analog `#defines` from Arduino.h in favor of pins_arduino.h
Remove Print.h
Merge pull request #2 from zfields/compat

Split Ethernet library into it's own NuGet package.