Added scan code enum to the keyboard driver source.

ISR:
*The keyboard ISR will now call the keyboard driver handler
function from the keyboard.h header (this mechanism will
probably be changed).

Keyboard:
*Created an enum with all the set of scan codes used by
the keyboard.
*Created an initial static structure that will hold the state of
the keyboard at all times.
*Minor refactoring of the keyboard port enums (added _ENUM at the
end of each enum name).