util.js: fix timer to not be global

Fix an issue with concurrent animations by making 'timer' not a global.
In further work, find out about JS's 'strict mode' to avoid this sort of
blunder.
Enable strict mode and fix up issues

Put the "use strict"; statement at the top of each .js source file to
catch silly mistakes.
Make js more jshint-friendly

Run JSHint over the .js source and fix up most complaints.
Use red and green to mark correctness of answers

Upon selecting an answer, highlight the correct answer in green. If
the chosen answer is incorrect then highlight that in red.