make the saving to python a method of the model, called with callbacks
make the saving to python a method of the model, called with callbacks

Conflicts:
	IPython/html/static/notebook/js/widgets/widget.js

cornhundred/ipywidgets,SylvainCorlay/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,SylvainCorlay/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets