Merge pull request #6 from gioele/documentation

Documentation fixes
Move KEYBOARD_MODEL include to implementation
Remove redundant function declarations
Use strbool instead of uint8_t
Raise error if there is no suitable impl of update_leds()
Use model dates in include guards
Merge pull request #7 from gioele/polish-hw-interface

Polish hw_interface
Fix encoding of Makefile
Raise an error if MODEL or LAYOUT are not set
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