Switched note sprites to their own class so I can change them later. Hopefully I haven't broken anything, but no way to know for sure: need to get some tests going.
Added keyboard input so you can now hit notes. While the note hits are based on timing and not on notes on the screen, the notes on the screen seem to drift slightly from the song over time. That's going to be an issue down the road, though I have an idea for a sync function that might quick fix it.