Add checking of only keys that changed

This allows key listeners to respond only if the specific key was
pressed or released on the last interrupt.  This should help prevent
missing key presses or detecting incorrect key presses.
Add a HighScoreScreen

This allows for the input of a players initials to be saved onto the SD
card.  Once the player inputs their initials the high scores are written
to the SD card and then shown to the player.
Add a more final looping Sound

This is a fairly short loop that works pretty well with the theme.
It'll suffice for testing, and may be kept for the final project.