UnderscoreSerializer: Resource names basic pluralization/configuration

By default all models will have their name underscored and pluralized by simply adding a 's' at the end. If someone ever want something else, it’s configurable by reopening the model’s class:

```javascript
App.MyModel.reopenClass({
  resourceName: 'my_special_models'
});
```
or

```javascript
App.MyModel.reopenClass({
  resourceName: function() {
    return 'my_special_models';
  }
});
```