Further cleanup of isEventSupported

I did a double-take on `isEventSupported` and realized that it could be further cleaned up.

It now performs all the tests directly on `document` instead of `document.createElement('div')`, unless we need to use the fallback method. In my tests, in all primary major browsers and IE8-11, this method has identical results to testing on a `<div>` for any event we are interested in and I don't see why it shouldn't.