Method to show hide linenumber of cell

mostly convenience methods to wrap codemirror one,
plus doc and 1 doc fix (missing curly bracket)

IPython/frontend/html/notebook/static/js/cell.js

@@ -226,6 +226,17 @@ var IPython = (function (IPython) {
         }
     };
 
+    /**
+     * Show/Hide CodeMirror LineNumber
+     * @method show_line_number
+     *
+     * @param value {Bool}  show (true), or hide (false) the line number in CodeMirror
+     **/
+    Cell.prototype.show_line_numbers = function (value) {
+        this.code_mirror.setOption('lineNumbers', value);
+        this.code_mirror.refresh();
+    };
+
     /**
      * Toggle  CodeMirror LineNumber
      * @method toggle_line_numbers

IPython/frontend/html/notebook/static/js/toolbar.js

@@ -40,7 +40,7 @@ var IPython = (function (IPython) {
      *          {
      *            label:'my button',
      *            icon:'ui-icon-disk',
-     *            callback:function(){alert('hoho'),
+     *            callback:function(){alert('hoho')},
      *            id : 'my_button_id',    // this is optional
      *          },
      *          {