Functionality to clear the notes [Fixes #8]

I had a similiar problem as described in #8.

Creating multiple BandJS (AudioContext) instances lead to

```
Uncaught SyntaxError: audio resources unavailable for AudioContext construction 
```

With this method I can now reset the notes and reuse the same BandJS instance, I am not sure if this clears everything, but at least it is working for me.