[requirejs] use requirejs over require, no simplified commonjs wrapper

* use requirejs over require variable name to avoid confusion on node.
  See jupyter/notebook#155 for details.

* don't use the requirejs simplified commonjs wrapper
  http://requirejs.org/docs/api.html#cjsmodule
  which is slower than a dependency array, and relies on toString parsing.
  See ipython-contrib/jupyter_contrib_nbextensions#1165
  for details.

main.js

@@ -1,11 +1,19 @@
-define(function (require, exports, module) {
+define([
+  'jquery',
+  'base/js/namespace',
+  'base/js/events',
+  'base/js/utils',
+  'notebook/js/codecell',
+], function (
+  requirejs,
+  $,
+  Jupyter,
+  events,
+  utils,
+  codecell
+) {
   "use strict";
-  var $ = require('jquery');
-  var Jupyter = require('base/js/namespace');
-  var events = require('base/js/events');
-  var utils = require('base/js/utils');
-  var CodeCell = require('notebook/js/codecell').CodeCell;
-
+  var CodeCell = codecell.CodeCell;
 
   var Scratchpad = function (nb) {
     var scratchpad = this;
@@ -124,7 +132,7 @@ define(function (require, exports, module) {
     var link = document.createElement("link");
     link.type = "text/css";
     link.rel = "stylesheet";
-    link.href = require.toUrl("./scratchpad.css");
+    link.href = requirejs.toUrl("./scratchpad.css");
     document.getElementsByTagName("head")[0].appendChild(link);
     // load when the kernel's ready
     if (Jupyter.notebook.kernel) {