fixed main.css

Removed Mixins portion, added a calc-grad-mid variable, removed old
background gradient, consolidated font-family, added a tag styling,
changed fixed height on .calc-header, altered padding on h1. nested
buttons into a .row selector, Added a keyframe for intro animation.
Changed index.html

I wrapped the buttons in rows.
Rewrote main.js

I ended up rewriting the main.js file completely.