Append a reference to the original object

data changes are not detected since `core-list` is cloning the original data and using it as the model of the template.

This pull request suggest a reference to the original data, so We can bind original properties to the template using an underscore like in the example.

Example:

```html
<core-list data="{{data}}" height="80">
  <template>
    <div class="item {{ {selected: selected} | tokenList }}"> {{_.name}} <br /> 
      <p><i>{{_.description}}</i></p> 
    </div>
  </template>
</core-list>
```

With this approach template is binding the original properties and can detect changes.