Scope/view access within event handlers

Includes the `Rivets.View` instance as the second argument in event handlers called from an `on-[event]` binding.

```javascript
controller = {
  removeItem: function(ev, view) {
    this // original event handler's context (likely the DOM node)
    ev   // original event handler's event object
    view // Rivets.View instance of the event binding

    items.remove(view.models.item)
  }
}
```

Closes #112, Closes #161