Support array, string or object in Model#unset

I found this to be useful for unsetting multiple attributes.

All of the following work the same:
```
model.unset('a1').unset('a2'); // fires 2 `"change"` events
model.unset(['a1', 'a2']); // fires 1 `"change"` event
model.unset({'a1': void 0, 'a2': void 0}); // fires 1 `"change"` event
```