dialog button handlers' "this" bound to dialog, not viewmodel

I see you've fixed up the events to get bound to the viewmodel, but the following binding does not work correctly:

```
<div data-bind="dialog: { isOpen: $root.isEditMode, buttons: { Save: $root.save} }"></div>
```

In the `save` function, `this` is bound to the jquery dialog, not the viewmodel. 
