add timer0 and filter non atmega328 code

added init for timer 0
added #ifdef guards around code that can only run on atmega2560
(temporary, now compiling on a atmega328)
makefile - add atmega2560 option
added some TODO items
init_io.h - add guards around define
testing output on arduino
added "void" in function declarations

"The void in the parenthesis are from C. In C a function with empty
parentheses could have any number of parameters. In C++ it doesn't make
any difference."

http://stackoverflow.com/questions/5587207/c-why-put-void-in-params