Merge pull request #9 from gioele/commented-main

Commented main
Define and document struct pin for AVR i/o
Document poll_timer_{enable,disable}
Fix documentation of poll_timer_setup
Document keyboard_init
Document the main function
Document the interrupt handler
Document the send function
Document the init function
Add minimal general documentation for main.c and hw_interface.c
Explain why we include KEYBOAD_MODEL_FILE
Generate tutorial from source code