Update main.js

This is a size improvement over repeated document.querySelector calls, aiming to better shrink once passed through minifiers.
The script is small already but being a base reference, why not smaller, if it's possible keeping it clean?
what matters is that you introduce a non semantic variable `$` that for developers means `jQuery` plus you are automatically cutting out iOS5 and other OS that have `document.querySelector` since ages but not `Function.prototype.bind`

Just don't overdo the basics ;-)