Remove the race condition with clearing the timers.  Because stopping the tone was based on a timeout.  Two consecutive notes could cause the first timer to clear, but then the callback would clear the next (wrong) timer.
Merging @BrianGenisio fix for `tone`.

Merge remote-tracking branch 'BrianGenisio/piezo-improvements' into ldg-piezo-play
Making the `tempo` attribute of `tune` on `Piezo.play` be a BPM tempo instead of ms indication. Tested.