Add fn to create and set custom Periodic Wave, add fn to schedule intervalic delay

You may not want to merge this one because it increases the size of the min file about .7k, but these are kind of cool features to create some interesting effects :) I would be curious if maybe you might have some ideas as to how to keep them implemented but maybe reduce the size back to what it was. Here's a rundown of size increases:

min.js: from 2.755 kb to 3.406 kb
.js: from 5.721 kb to 7.665 kb

Thanks for putting this lib together, it's an awesome interface to the WebAudio API!