Piezo fixes for #413

`Piezo.prototype.tone` will not try to play `NaN` tones anymore, preventing piercing grief.

Note that the tests I added couldn't include the original offending wonky array syntax; `jshint` won't allow it. I did add a test for it at one point and ran the `nodeunit` grunt task by itself and it passed.
(mans store it on the piezo instance)