Preserve original constructor name

This improves the debugging output since you can display the class name instead of just Model/View/Collection by adding a named constructor like this:

```js
constructor: function MyModel() {
  Backbone.Model.apply(this, arguments);
 },
```
The debug output looks more like:
```
[instance] Model: MyModel:c6
```
This also improves compatibility with the [Backbone-Debugger](https://github.com/Maluen/Backbone-Debugger/) chrome extension.