move button functionality to external javascript

In the name of unobtrusive JS (separation of behaviour from markup or
structure), separated the onclick and javascript:void(0) link from the
HTML into the external JS file.

- consolidate button from to single <div> (index.html)

- remove inline javascript (index.html)

- change a.exercise styles to .button.exercise (css/main.css)

- encapsulate button set into function (js/app.js)