Add option to use a callback to retrieve a model or collection from the component’s props

This feature allows you to specify a callback to retrieve the Backbone model or collection to listen to.

Some of our models have a model defined as a property. This method allows you to listen for changes on those models as well.

Example:

```javascript
var CommentView = React.createBackboneClass({
    mixins: [
        React.BackboneMixin("comment"),
        React.BackboneMixin(function(props) {
            return props.comment.user;
        })
    ],
    render: function() {
        return (
          <div>
              <p>{this.props.comment.get("text")}</p>
              <p>{'posted by' + this.props.comment.user.get("name")}</h1>
          </div>
        );
    }
});
```