Event Handler Configuration

Adds a configuration option for [event handler APIs](https://github.com/mikeric/rivets/issues/161#issuecomment-18307472). Like all other configuration, this can be set globally on `rivets.config` or locally to a particular `Rivets.View` instance.

The default event handler configuration passes one extra object to the handler — the `view.models` object. Here's an example of what an event handler would look like for the default configuration.

```javascript
completeItem: function(ev, models) {
  models.item.set({completed: true})
}
```

To change this API, you can overwrite the `rivets.config.handler` function.

```javascript
rivets.config.handler = function(fn, context, ev, binding) {
  fn.call(context, ev, binding.model, binding.view)
}
```

Closes #161