get cell correctly in nested context

fix some extensions like live_reveal

the .not('.cell .cell') part prevent the potential selection of DOM
elelment that would have the class cell from any output. In each branch
of the dom we thus select the first decendent node that have the class
'.cell'.

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

@@ -379,7 +379,7 @@ define([
      * @return {jQuery} A selector of all cell elements
      */
     Notebook.prototype.get_cell_elements = function () {
-        return this.container.children("div.cell");
+        return this.container.find(".cell").not('.cell .cell');
     };
 
     /**