Simple binding with Select2 data object option

I have changed the directive to always bind ng-model to the simple value returned by select2. I have also added a ui-data attribute that enables you to bind a property in your controller to the select2 data object if you need to access it. 

Lets say select2 returns the object 

```javascript
{ id: 1, text: "My Selection" }
```
ng-model will equal 1
ui-data will equal 

```javascript
{ id: 1, text: "My Selection" }
```

This addresses the following issue

[https://github.com/angular-ui/ui-select2/issues/126](https://github.com/angular-ui/ui-select2/issues/126)