Library rewrite

NOTICE: Since this version library structure has had important changes. You need to make minor changes in your code but IDE will refuse to compile until you do it. Check latest included example.

Add Arduino boards support via preprocessor

I've added Arduino boards support via preprocessor directives. Currently only W5100 ethernet, not Arduino WiFi yet.

I've checked that example compiles but I've had not time to test that it works. Code is simple so it should work.

Initial version on Arduino repository

1.2