Valid event

It would be nice if the view emitted a `valid` event whenever it's `valid` property changed.
Right now, I have something like this:

```
FormView.extend({
    validCallback: function(valid) {
        this.trigger("valid", valid);
    },
})
```

However, it's a pain to add it to all my forms, and I fires every time (vs. just when it changes).
