Cherry pick of @jasongrout 's 2bc2f96.

Only merged changes to notebook and kernel...  (see original comment below).

Separate the widget registry from the widget managers;
attach widget managers to kernels.

Yet to do: take care of clean-up when a kernel is deleted
Conflicts:
	IPython/html/static/notebook/js/widgetmanager.js
	IPython/html/static/notebook/js/widgets/widget.js

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

@@ -1315,7 +1315,6 var IPython = (function (IPython) {
      */
     Notebook.prototype._session_started = function(){
         this.kernel = this.session.kernel;
-        IPython.widget_manager.attach_comm_manager(this.kernel.comm_manager);
         var ncells = this.ncells();
         for (var i=0; i<ncells; i++) {
             var cell = this.get_cell(i);

IPython/html/static/services/kernels/js/kernel.js

@@ -47,6 +47,9 var IPython = (function (IPython) {
         this.bind_events();
         this.init_iopub_handlers();
         this.comm_manager = new IPython.CommManager(this);
+        // TODO: make the comm manager an arg to the widget manager initialization
+        this.widget_manager = new IPython.WidgetManager();
+        this.widget_manager.attach_comm_manager(this.comm_manager);
     };