adding a tooltip in IPython buttons

IPython/html/static/widgets/js/widget_button.js

@@ -12,7 +12,7 b' define(['
             // Called when view is rendered.
             this.setElement($("<button />")
                 .addClass('btn btn-default'));
-
+            this.$el.attr("data-toggle", "tooltip");
             this.model.on('change:button_style', function(model, value) {
                 this.update_button_style();
             }, this);
@@ -27,6 +27,7 b' define(['
             // Called when the model is changed.  The model may have been 
             // changed by another view or by a state update from the back-end.
             var description = this.model.get('description');
+            this.$el.attr("title", this.model.get("tooltip"));
             if (description.length === 0) {
                 this.$el.html("&nbsp;"); // Preserve button height
             } else {

IPython/html/widgets/widget_button.py

@@ -29,7 +29,8 b' class Button(DOMWidget):'
     _view_name = Unicode('ButtonView', sync=True)
 
     # Keys
-    description = Unicode('', help="Description of the button (label).", sync=True)
+    description = Unicode('', help="Button label.", sync=True)
+    tooltip = Unicode(help="Tooltip caption of the button.", sync=True)
     disabled = Bool(False, help="Enable or disable user changes.", sync=True)
 
     button_style = CaselessStrEnum(