Not setting tags explicitly

Not setting tags explicitly as that circumvents ng-model and can create invalid models on parent, especially when model hasn't been loaded yet (tags === undefined)

Consider the following scenario
```html
<tags-input ng-model="someObject.metadata.tags"></tags-input>
```

With the previous code, if ```someObj``` was undefined/null, tagsInput would create the whole ```someObj.metadata``` objects as empty objects with one property ```tags``` that would be an empty array. With the only seeming reason being that you can count on running ```scope.tags.length```.  This could invalidate any validations on the parent. Allowing for ```scope.tags``` to be undefined/null should be up to the consumer, and the value shold only be set through ```$ngModelCtrl.setViewModel()```