Add a requirejs shim for highlight.js

Before this, highlighting code in markdown cells was broken since hljs wasn't available.

A better solution might be to build the highlight.js amd module (i.e., build the highlight.js module with `--target=amd`) so that we don't need the shim.

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

@@ -1,6 +1,12 b''
 // Copyright (c) IPython Development Team.
 // Distributed under the terms of the Modified BSD License.
 
+requirejs.config({
+    shim: {
+	'components/highlight.js/build/highlight.pack': {exports: 'hljs'}
+    }
+});
+
 require([
     'base/js/namespace',
     'jquery',

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

@@ -11,6 +11,7 b' define(['
     'services/sessions/js/session',
     'notebook/js/celltoolbar',
     'components/marked/lib/marked',
+    'components/highlight.js/build/highlight.pack',
     'notebook/js/mathjaxutils',
     'base/js/keyboard',
     'notebook/js/tooltip',
@@ -27,6 +28,7 b' define(['
     session, 
     celltoolbar, 
     marked,
+    hljs,
     mathjaxutils,
     keyboard,
     tooltip,