Detect vendor prefixes more sanely.

Previously it would pick the prefixed version for Chrome, even when the unprefixed was available. Also use the transform and not the transition property for testing, since safari still doesn't support unprefixed transform but does support unprefixed transition.
`event` feels like a misnomer now that you've changed the `vendors` object. Before it was referring to the prefix for the event name. Now, maybe you could call the local variable simply `vendor`?