enable comment/uncomment selection

with `(cmd|ctrl)-/`

closes  #1540

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

@@ -85,7 +85,13 b' var IPython = (function (IPython) {'
 
     CodeCell.options_default = {
         cm_config : {
-            extraKeys: {"Tab": "indentMore","Shift-Tab" : "indentLess",'Backspace':"delSpaceToPrevTabStop"},
+            extraKeys: {
+                "Tab" :  "indentMore",
+                "Shift-Tab" : "indentLess",
+                "Backspace" : "delSpaceToPrevTabStop",
+                "Cmd-/" : "toggleComment",
+                "Ctrl-/" : "toggleComment"
+            },
             mode: 'ipython',
             theme: 'ipython',
             matchBrackets: true

IPython/html/templates/notebook.html

@@ -211,6 +211,7 b' class="notebook_app"'
 <script src="{{ static_url("components/codemirror/addon/mode/multiplex.js") }}" charset="utf-8"></script>
 <script src="{{ static_url("components/codemirror/addon/mode/overlay.js") }}" charset="utf-8"></script>
 <script src="{{ static_url("components/codemirror/addon/edit/matchbrackets.js") }}" charset="utf-8"></script>
+<script src="{{ static_url("components/codemirror/addon/comment/comment.js") }}" charset="utf-8"></script>
 <script src="{{ static_url("notebook/js/codemirror-ipython.js") }}" charset="utf-8"></script>
 <script src="{{ static_url("components/codemirror/mode/htmlmixed/htmlmixed.js") }}" charset="utf-8"></script>
 <script src="{{ static_url("components/codemirror/mode/xml/xml.js") }}" charset="utf-8"></script>