Added more GUI controls, added work to add images

Now can start and stop the application, and control the stipple size. An
image can be selected from the harddrive, currently it is just displayed,
and its pixels will eventually be read. Next step is to somehow resize the
canvas to match the selected image.

Worked on #3, #5, #9

	modified:   .gitignore
	modified:   index.html
	modified:   voronoi.js