inject_script.js

var inject_script = function(source) {
    if (!source) source = "//cdnjs.cloudflare.com/ajax/libs/jquery/1.9.1/jquery.js";
    
    var script = document.createElement("script");
    script.src = source;
    script.type="text/javascript";
    
    document.getElementsByTagName("head")[0].appendChild(script);
    
    var listen = setInterval(function() {
        if(typeof jQuery !== 'undefined') {
            $ = jQuery;
            clearInterval(listen);
        }
    },50);
};

inject_script();