Add user-visible note that IE<=8 lacks <canvas>

There was no point pretending to support IE<=8
when they can't work for the entire point of this
page.  Better to try and let the user know.
Change JS build system.

The new system is a bit more efficient and replicable.
Stop requiring localStorage/cookies.

Every likely browser that supports canvas supports localStorage,
but using localStorage never served a purpose and made it harder
for users that disable cookies and similar tech (localStorage
being similar).
Move images to a subdirectory.

The script dynamically constructs a URL to find
some of these images, and I think it's a bit clearer
with them in a subdirectory.  Also it helps my
website infrastructure deal with that nicely.
update README
Rename playground.html to index.html
add informational comment in build script