function addScript()
		{
			var script = document.createElement('script');
			script.type = 'text/javascript';
			script.src = url.shift(1);
			script.onload = function(e) { 
				if(url.length)
				{
					if(on_progress)
						on_progress(url[0], url.length);

					addScript();
					return;
				}

				loaded_scripts.push(this);
				
				if(on_complete)
					on_complete( loaded_scripts );
			};
			document.getElementsByTagName('head')[0].appendChild(script);
		}
Request script and inserts it in the DOM
@method requireScript
@param {String|Array} url the url of the script or an array containing several urls
@param {Function} on_complete
@param {Function} on_error
@param {Function} on_progress (if several files are required, on_progress is called after every file is added to the DOM)