Lazy load JS files

common.js

function load_script(src, callback) {
	var head = document.getElementsByTagName('head')[0];
	var script = document.createElement('script');

	script.type = 'text/javascript';
	script.src = src;

	script.onload = callback;
	script.onreadystatechange = function() {
		if (this.readyState == 'loaded' || this.readyState == 'complete') {
			callback();
		}
	}

	head.appendChild(script);
};