Merge pull request #8 from gioele/makefile

Makefile
Use main.commented as main.c
Use the more descriptive KEYBOARD_MODEL_FILE
Use MAX_PKEYS constant instead of magic number 6
Use NO_KEY constant instead of magic number 255 for key queue
Document NO_KEY and split queue documentation
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