Repeated calls to tone() breaks on tiny25

```
Tone() seems to need at least 5ms between each call, or there is no output.

ex:
loop(){
tone(1, 440);
}

gives no output, but

loop(){
tone(1, 440);
delay(10);
}

gives a stable 440Hz output. 10ms seems to be a good figure, it will work at 
5ms, but the output is unstable.
```

Original issue reported on code.google.com by `martin.b...@gmail.com` on 21 Sep 2014 at 2:39
