Version 0.3: Add a GUI for building SVGs.

New class SVGBuilderWidget for drawing new elements in the SVG. There
is a ToggleButtonsWidget to select the drawing mode, which attaches
different event listeners in Javascript for drawing the elements.

	modified:   .gitignore
	modified:   svgwidgets.js
	modified:   svgwidgets.py
	modified:   svgwidgets_example.ipynb