[bugfix] Use CustomEvent instead of Event

```js
ImgCache.init()
TypeError: Illegal constructor
```

It's stack trace leads to this line:
```js
DomElement.dispatchEvent(new Event(eventName));
```

I don't know if it's case with every Cordova device, but on my Cordova 3.3 and Android `new Event('foo')` raises a Error, while `new CustomEvent('foo')` is okay.