load the per kernel kernel.js and kernel.css

As per discussion, each kernel can provide a file name kernel.js that
we try to load at kernel switching. If such a file exist we assume that
the kernel pathches the javasscript and that this javascript cannot be
unpatched, and further switching of the kernel cannot be undone without
reloading the page. (separate PR for UI)

if a kernel provide kernel.js, the it should consist into a AMD module
definition that uses require.js the module shoudl define a function name
`onload` that will be called at the appropriate moment before the kernel
starts.

IPython/html/static/notebook/js/kernelselector.js

@@ -21,6 +21,7 b' define(['
         this.bind_events();
         // Make the object globally available for user convenience & inspection
         IPython.kernelselector = this;
+        Object.seal(this);
     };
     
     KernelSelector.prototype.request_kernelspecs = function() {
@@ -61,10 +62,30 b' define(['
     };
 
     KernelSelector.prototype.change_kernel = function(kernel_name) {
+        /**
+         * TODO, have a methods to set kernel spec directly ?
+         **/
+        var that = this;
         if (kernel_name === this.current_selection) {
             return;
         }
         var ks = this.kernelspecs[kernel_name];
+        var new_mode_url = 'kernelspecs/'+ks.name+'/kernel';
+
+        var css_url = this.notebook.base_url+new_mode_url+'.css';
+        $.ajax({
+            type: 'HEAD',
+            url: css_url,
+            success: function(){
+                $('#kernel-css')
+                .attr('href',css_url);
+            },
+            error:function(){
+                console.warn(new_mode_url+' does not provide custom URL, you might see a 404 error that shoudl not prevent '+
+                                     ' the Jupyter notebook from working :' ,css_url );
+            }
+        });
+
         try {
             this.notebook.start_session(kernel_name);
         } catch (e) {
@@ -78,6 +99,30 b' define(['
             return;
         }
         this.events.trigger('spec_changed.Kernel', ks);
+
+
+        // load new mode kernel.js if exist
+        require([new_mode_url],
+            // if new mode has custom.js
+            function(new_mode){
+                that.lock_switch();
+                if(new_mode && new_mode.onload){
+                    new_mode.onload();
+                } else {
+                    console.warn("The current kernel seem to define a kernel.js file; though this file does"+
+                                 "not contain any asynchronous module definition. This is undefined behavior"+
+                                 "which is not recommeneded");
+                }
+            },
+            function(err){
+                // if new mode does not have custom.js
+                console.warn('Any above 404 on '+new_mode_url+'.js is normal');
+            }
+        );
+    };
+
+    KernelSelector.prototype.lock_switch = function() {
+        console.warn('switching kernel is not guarantied to work !');
     };
     
     KernelSelector.prototype.bind_events = function() {

IPython/html/static/notebook/js/main.js

@@ -113,6 +113,7 b' require(['
     notification_area.init_notification_widgets();
     var kernel_selector = new kernelselector.KernelSelector(
         '#kernel_selector_widget', notebook);
+    notebook.set_kernelselector(kernel_selector);
 
     $('body').append('<div id="fonttest"><pre><span id="test1">x</span>'+
                      '<span id="test2" style="font-weight: bold;">x</span>'+

IPython/html/static/notebook/js/notebook.js

@@ -1864,7 +1864,14 b' define(['
         // Trigger an event changing the kernel spec - this will set the default
         // codemirror mode
         if (this.metadata.kernelspec !== undefined) {
-            this.events.trigger('spec_changed.Kernel', this.metadata.kernelspec);
+            // TODO shoudl probably not trigger here, 
+            // should call the kernel selector, or custom.{js|css} not loaded.
+            if(this.kernel_selector){
+                // technically not perfect, we should check that the kernelspec matches
+                this.kernel_selector.change_kernel(this.metadata.kernelspec.name);
+            } else {
+                console.log('do not have handle on kernnel_selector');
+            }
         }
         
         // Set the codemirror mode from language_info metadata
@@ -2308,6 +2315,10 b' define(['
         this.events.trigger('notebook_loaded.Notebook');
     };
 
+    Notebook.prototype.set_kernelselector = function(k_selector){
+        this.kernel_selector = k_selector;
+    };
+
     /**
      * Failure callback for loading a notebook from the server.
      * 

IPython/html/templates/notebook.html

@@ -17,6 +17,7 b' window.mathjax_url = "{{mathjax_url}}";'
 {{super()}}
 
 <link rel="stylesheet" href="{{ static_url("notebook/css/override.css") }}" type="text/css" />
+<link rel="stylesheet" href=""  id='kernel-css'                             type="text/css" />
 
 {% endblock %}
 

IPython/html/templates/page.html

@@ -24,6 +24,7 b''
           baseUrl: '{{static_url("", include_version=False)}}',
           paths: {
             nbextensions : '{{ base_url }}nbextensions',
+            kernelspecs : '{{ base_url }}kernelspecs',
             underscore : 'components/underscore/underscore-min',
             backbone : 'components/backbone/backbone-min',
             jquery: 'components/jquery/jquery.min',