Adding README
Early refactor work for audio playback fixes

So far the major changes have been around the loop/instrument models. It turns out, we need to create a new AudioBufferSourceNode every time we want to play a sound. It can only play once. This refactors things so that the InstrumentModel stores its own buffer, but the audio service handles playback by creating new sources when play() is called.

I'm also renaming the little buttons to beat buttons but that's next.
Beats in place

Using BeatCollection, BeatModel, etc. A beat is essentially a button on screen. And the number that shows depends on the total beats per song