Check if CustomEvent is defined, not if it is a function

Safari for Mac (6.0.5) and Safari for iOS (6.1) both return
"object" for "typeof CustomEvent". As such, Tap.js previously
defaulted to the deprecated document.createEvent mechanism
event though Safari supports the new API.

![screen shot 2013-07-26 at 10 16 37](https://f.cloud.github.com/assets/243719/861296/3d2f11d6-f5cc-11e2-98f5-8eff589e6a91.png)