Added an option to disable CSS transitions

I added this option as there is a bug in Firefox where using CSS transitions on an element that contains flash will cause the flash not to appear. Whilst it can easily be fixed if you control the flash, you can't do anything if your flash is within an embed code.

Details on the firefox bug here:
https://bugzilla.mozilla.org/show_bug.cgi?id=819763#c0

If the useCSSTransitions option is set to true (which it is by default) it still does feature sniffing to determine if it can be done. So this will not affect anyone's existing implementations.