Add results animation, more content to index.js.

Add:
    static/js/index.js
        -Add `populateResults()`, to distinguish the addition of results to
        `div#results` from the retrieval of Codelets from the server itself.
        -Add `clearResults()` for convenience.

    static/sass/index.sass
        -Add `.cascade`, to provide new `div.result` elements with a
        downwards-cascading animation.