Add warning to widget namespace import.

IPython/html/widgets/__init__.py

@@ -22,3 +22,8 b' from .widget_int import IntTextWidget, BoundedIntTextWidget, IntSliderWidget, In'
 from .widget_selection import RadioButtonsWidget, ToggleButtonsWidget, DropdownWidget, SelectWidget
 from .widget_selectioncontainer import TabWidget, AccordionWidget
 from .widget_string import HTMLWidget, LatexWidget, TextWidget, TextareaWidget
+
+# Warn on import
+from IPython.utils.warn import warn
+warn("""The widget API is still considered experimental and 
+    may change by the next major release of IPython.""")