Add support for more complicated keyboard scan codes

[jordanhendricks]

#211 implements a basically-fully-functional ps/2 keyboard, but there are some gaps in scan code handling. Part of the differences here were driven by a desire for backwards compatibility between different sizes of keyboards.

Such gaps include:

• the Pause key (a 4-byte scan code) is not yet implemented (for Set 1 or Set 2)
• Some keys, such as Insert and Delete, will send different scan codes, depending on whether Shift is down or NumLock is on.