Require *both* 64 and -64 to win

- Track wonHigh and wonLow in game state.
- Update index.html and README.md for the new win condition.

Technically, one could win without having both at the same time, e.g.
reaching all the way to 128 and then making a -64.  Seems unlikely to
happen, but that's still plenty deserving of a win.
Merge branch 'master' into gh-pages
update cache.appcache date