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?
> Sorry I didn't mean to be rude

You weren't. Don't read emotions into text on the internet :D

> No need to cut them out because of .bind that brings nothing in this very specific snippet.

You can easily shim .bind too if you need to support older browsers. Like you said we already use other things that won't work in iOS 5.

- [x] .bind support: https://github.com/es-shims/es5-shim/

We're not going to limited ourselves to ES3 just because of browsers we don't support.