Ability to cancel future events on return false / Avoid generating keyframe selectors with same id

[riyazshaikh]

A couple of fixes/enhancements I made to make your code work for my usage.

[csuwildcat]

I have added a simple counter to ensure that every listener has a different ID. This should cover half this pull request. I still don't feel right to change what return false does due to the behavior of native DOM APIs folks are already used to.