ng-cookie directive

```
<input ng-model="name" ng-cookie="name">
```
This persists `$scope.name` to cookie['name'] whenever the model changes, deletes the cookie when the model goes undefined, and sets the initial value of `$scope.name` to cookie['name']. 

(The cookie name is isn't generated from the model name because cookies have bigger scope (domain, path) than models (controller).)