Add static file handler

ipython_widgets/widget_handler.py

@@ -0,0 +1,12 @@
+import os
+from IPython.html.utils import url_path_join as ujoin
+from tornado.web import StaticFileHandler
+
+module_path = os.path.dirname(__file__)
+def load_jupyter_server_extension(nbapp):
+    webapp = nbapp.web_app
+    base_url = webapp.settings['base_url']
+    webapp.add_handlers(".*$", [
+        (ujoin(base_url, r"/widgets/static/(.+)"), StaticFileHandler, 
+            {'path': os.path.join(module_path, 'static')}),
+    ])