Refactor KeyboardLayoutUS to allow other layouts without duplicating code

[Dan Halbert]

Factor out the common part of KeyboardLayoutUS into a common superclass, so that someone could add another keyboard layout just by specifying the ASCII (or Unicode?) to keycode table, and sharing the rest of the code. See #46 for the initial motivation for this.

[Angainor Development]

First attempt here #61
Comments are welcome.

[Dan Halbert]

Fixed by #84.