Support for ngModel

Currently, changed values can only be recognized using a `value-changed` callback.
Implement a `ng-model` binding to allow:

```
<fancy-select ng-model="data"/>
```

with data being e.g.:

```
{value: [], possibleValues: [{caption: "Option A", id: 5}, {caption: "Option B", id: 7}]}
```

and internally doing e.g.:
`scope.items=data.possibleValues`
`scope.value=data.value`
