Align colons in help dialog.

IPython/frontend/html/notebook/static/css/notebook.css

@@ -311,3 +311,11 b' div.text_cell_render {'
     .ui-widget-content { border: 0px; }
 }
 
+.shortcut_key {
+    display: inline-block;
+    width: 10ex;
+    text-align: right;
+}
+
+.shortcut_descr {
+}
  No newline at end of file

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

@@ -201,8 +201,8 b' var IPython = (function (IPython) {'
         ];
         for (var i=0; i<shortcuts.length; i++) {
             dialog.append($('<div>').
-                append($('<span/>').addClass('shortcut_key').html(shortcuts[i].key+' : ')).
-                append($('<span/>').html(shortcuts[i].help))
+                append($('<span/>').addClass('shortcut_key').html(shortcuts[i].key)).
+                append($('<span/>').addClass('shortcut_descr').html(' : ' + shortcuts[i].help))
             );
         };
         dialog.dialog({title: 'Keyboard shortcuts'});