Default value of paramAttributes options

If a `paramAttributes` option does not exists on the element, its default value is `0`:

http://jsfiddle.net/H5BX5/

I think it should be `null`, it would allow to easily define default options:

``` js
Vue.component({
    paramAttributes: ['bar'],
    created: function() {
        if (this.bar === null) this.bar = 'default value'
    }
})
```
