Exceptional handling for PS/2 scan code set 2

changed names which does not comply to C spec.(underscore prefix names)

rewrite code of layer switching

PS/2 to USB keyboard converter

added description of ADB socket pintouts.

changed wait time for volume control.

changed special mode key for macway: Left Shift + Right Shift

ADB keyboard LEDs support

added 'Keymap' section to adb/README.

change keycodes and define keymap macro for AEK.

ADD: keymap macro for Apple Extended Keyboard in adb/keymap.c
FIX: keycodes for Keypad, Fn and Mousekey.

ADB to USB keyboard converter

FIX: error handling in ps2.c

added a file TODO.

changed signature of keymap_fn_layer() in keymap_skel.h.

FIX: name of mousekey macros usb_keycodes.h.

added config option: MATRIX_HAS_GHOST and fixed some on matrix.c

ADD: Build option: MATRIX_HAS_GHOST to enable ghost blocking logic.
FIX: choose matrix buffer type(uint8_t/uint16_t) automatically
     depending on column size in matrix.c.
FIX: use uint8_t insted of int in matrix.c.

add "Build your own firmware" and "Features" section to README.

add a build option: USB_EXTRA_ENABLE

add error handling to ps2_mouse

Add PS/2 mouse support to connect TrackPoint Unit.

Change build options:  Makefile and config.h. See README.

change keyboard report descriptor for NKRO.
It uses 1byte for modifiers and 15bytes(120bits) for keys now.

add build option: NKRO_ENABLE(remove: USB_12KRO)