Register init_widget_js

and remove cls parameter

IPython/html/widgets/__init__.py

@@ -1,4 +1,4 b''
-from widget import Widget
+from widget import Widget, init_widget_js
 
 from container import ContainerWidget
 from float_range import FloatRangeWidget

IPython/html/widgets/widget.py

@@ -13,7 +13,7 b' from IPython.display import Javascript, display'
 from IPython.utils.py3compat import string_types
 from IPython.utils.javascript import display_all_js
 
-def init_widget_js(cls):    
+def init_widget_js():    
     path = os.path.split(os.path.abspath( __file__ ))[0]
     display_all_js(path)
     for root, dirs, files in os.walk(path):