Fix second stylus button and eraser on Graphire

The Graphire has a second styles button using bit 2 (mask 0x04) of the button-
bits, because of this it also has the eraser moved to bit 3 (mask 0x08).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Add a TODO file

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Add protocol documentation from linuxwacom-0.9.9

Signed-off-by: Hans de Goede <hdegoede@redhat.com>