script.user.js

// ==UserScript==
// @name Userscript Name
// @namespace http://example.com/
// @description Userscript Description
// @match http://example.com/* (or @include * to include all pages)
// @version 1.0
// ==/UserScript==

// Inject JavaScript onto the page
function exec(fn) {
    var script = document.createElement('script');
    script.textContent = '(' + fn + ')();';
    document.head.appendChild(script);
}

// Include external JS/CSS file onto the page
function include(url, type) {
    type = (type || url.split('.').pop()).toLowerCase();
    
    if (type === 'css') {
        var link = document.createElement('link');
        link.rel = 'stylesheet';
        link.href = url;
        document.head.appendChild(link);
    } else if (type === 'js') {
        var script = document.createElement('script');
        script.src = url;
        script.async = false;
        document.head.appendChild(script);
    } else {
        throw new Error('Failed to include ' + url + ' due to unknown file type.');
    }
}​

// Executes function asynchronously 
function async(fn) {
    setTimeout(fn, 0);
}

// include jquery if it doesn't already exist
if (typeof jQuery == 'undefined') {
  include('https://code.jquery.com/jquery-2.1.3.min.js', 'js');
}

// include javascript or css here...
// include('https://rawgit.com/username/project/master/file.js', 'js');
// include('https://rawgit.com/username/project/master/file.css', 'css');