Merge pull request #365 from mithro/firefox-fixes

Fixes to web-animations.js for Firefox/Chrome differences.
Don't fail to keep browser running on an exception.
Deal with Firefox's "Your script is taking too long" type alerts.
Use setTimeout to move to the next event to stop Firefox being unhappy.
Use a different method of hidding the elements which mean getComputedStyle still works.
Make the browser maximized and screenshots of failures nicer.
Ignore the png files generated by run-tests when tests fail.
Output a better error on bad JSON.
Give the browser a moment to scroll before taking screenshot.