pumpit.js

var queue = [];
var scripts ={};

function loadIt(scriptname) {
  queue.push(scriptname);
  if (!scripts[scriptname]) {
    var el  = document.createElement('script');
    el.src = scriptname;
    document.head.appendElement(el);
  } else pump();
}

function fullfill(scriptname, script) {
  scripts[scriptname] = script;
  pump();
}

function pump() {
  if (queue.length === 0) return; // nothing to do
  var name = queue[0];
  if (scripts[name]) {
    queue.shift(); // remove item
    eval(scripts[name]);
    setTimeout(pump, 1); // try to pump again.  take a break to avoid slow scripts
}