Support passing a callback to "add"

[weotch]

This adds support for passing a callback to add. My use case is wanting to use this library with Vue.js where I want to use Vue to set style attributes on the parallaxed element. I'm using like:

var parallaxRoot = parallax.create(domElement);
parallaxRoot.add(domElement, function(ratio) {
  // Store the ratio in the Vue component
});

Also, this PR adds a destroy() method.