Convert to "CommonJS style" AMD

ReWrite the `define` factory functions to accept an argument named
`require`, and use it as though it synchronously loaded modules (just
like in CommonJS). This makes modules slightly more maintainable since
dependencies may be added and removed without the need to synchronize
two separate-but-related lists.

src/main.js

@@ -1,5 +1,7 @@
-define(['modules/photo', 'modules/view'], function(Photo, PhotoView) {
+define(function(require) {
   'use strict';
+  var Photo = require('modules/photo');
+  var PhotoView = require('modules/view');
 
   var me = new Photo({
     id : 19

src/modules/photo.js

@@ -1,5 +1,6 @@
-define(['backbone'], function(Backbone) {
+define(function(require) {
   'use strict';
+  var Backbone = require('backbone');
 
   var Photo = Backbone.Model.extend({
     urlRoot : function() {

src/modules/view.js

@@ -1,5 +1,8 @@
-define(['backbone', 'jquery', 'underscore'], function(Backbone, $, _) {
+define(function(require) {
   'use strict';
+  var Backbone = require('backbone');
+  var $ = require('jquery');
+  var _ = require('underscore');
 
   var PhotoView = Backbone.View.extend({