add create.Cell and delete.Cell js events

The events fire after the cell has been created or deleted.

closes  #3154

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

@@ -737,6 +737,7 b' var IPython = (function (IPython) {'
                 this.undelete_index = i;
                 this.undelete_below = false;
             };
+            $([IPython.events]).trigger('delete.Cell', {'cell': cell, 'index': i});
             this.set_dirty(true);
         };
         return this;
@@ -777,6 +778,7 b' var IPython = (function (IPython) {'
             if(this._insert_element_at_index(cell.element,index)){
                 cell.render();
                 this.select(this.find_cell_index(cell));
+                $([IPython.events]).trigger('create.Cell', {'cell': cell, 'index': index});
                 this.set_dirty(true);
             }
         }