Add item view attributes for collection view items

I added a way to pass attributes to the constructor of the item view.

This is how it is used Inside a collection view:
```javascript
Thorax.CollectionView.extend({
    name: "app/list",
    initialize: function() {
        this.itemAttributes = {
            myVar : 'value'
        }
});
```

Then inside a collection item, myVar is availble:
```javascript
Thorax.View.extend({
    name: "app/list-item",    
    initialize: function() {
        var a = this.myVar;
    }
});
```