AVR-LIBC time.h

I have some ideas for 'easy' integration of the AVR-LIBC time functions into the Arduino 'runtime'. This would give the Arduino system a 'platform standard' time keeping system, beneficial in itself, and also C-90 compliant.

I have already implemented these ideas in the 1.6.5 version, simply by modifying wiring.c to re-define timer0_millis as uint64_t (along with the temporary m), and then a new file 'arduino_time.c', to obtain time() from timer0_millis... essentially time() is just millis()/1000.

A downside is that this pushes a few clock cycles onto the timer0 ISR.

Upside is that it is all transparent to older code.