Trigger layout invalidation on widget display

[peytondmurray]

Summary

This PR triggers layout invalidation when the widget is displayed, closing #58. See that issue for a complete discussion and examples.

Changes

• This change modifies how ipywidgets are displayed by allowing an onDisplay callback to be passed to the WidgetManager.render function which triggers when the view's displayed event is emitted.
• Back in the IPyWidgetView, the widget manager's render function is passed a callback which invalidates the layout.
• Also modified the "dev" mode script to avoid minimizing the bundle, making debugging easier.

[peytondmurray]

Sounds like while this solves the minimal working example, it doesn't address all use cases. Moving back into draft.