Remove update-preview button and make it run smoothly

Implement redraw asynchronously using $interval; use generators for a
uniform chunked interface to drawing.

Make drawPenStroke{Outline,Centerline} into private methods (prefixed
with underscore).
Provide ES6→5 translation for node and disadvantaged browsers
Merge pull request #191 from rrthomas/smooth-red-pill

Smooth red pill