Fix event functions

jQuery like event functions is defined in jquery.d.ts as follows:

```js
on(events: string, handler: (eventObject: JQueryEventObject, ...args: any[]) => any): JQuery;
```
https://github.com/borisyankov/DefinitelyTyped/blob/master/jquery/jquery.d.ts

And they are used in zepto official tests as follows:

```js
this.el.on('click', function(){ numArgs = arguments.length })
```
https://github.com/madrobby/zepto/blob/master/test/event.html

Therefore, I think that this definitions is better.