Refactored buttons controller to an attribute on json-editor

- Can now specify controller per json-editor element. its dynamically created and injected the directive's scope.
- Removed `controller` from config constant.

Buttons/actions controller per editor is specified like this.
```html
<json-editor ... buttons-controller="ButtonsController1">

    <button ng-click="onButton1()">Button 1</button>

</json-editor>
```

```javascript
...controller('ButtonsController1', function ($scope) {
    $scope.onButton1 = function () {
        console.log('Button 1 Clicked');
    };
})
```