Forgot to checkin new ResourcesDict class...

IPython/nbconvert/exporters/exporter.py

@@ -70,6 +70,11 b' default_filters = {'
 # Class
 #-----------------------------------------------------------------------------
 
+class ResourcesDict(collections.defaultdict):
+    def __missing__(self, key):
+        return ''
+
+
 class Exporter(Configurable):
     """
     Exports notebooks into other file formats.  Uses Jinja 2 templating engine