VGA is finally usable for something like debugging.

VGA:
*Implemented the missing string and clear screen functions.
*The implementation of the planned functions for the 25x80 VGA
text mode is done. It seems all of the functions work fine (the
clear_screen seems a bit slow). For more info check the comments
in the text_mode.c source.

Other:
*Added proper comments to port_io.c.
*The kernel tests printing strings and clearing the screen.